JP2005294139A - Lithium-ion secondary battery and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve a problem that a discharge characteristic is reduced in a lithium-ion secondary battery in which a heat resistant porous film layer is formed on an electrode plate for the purpose of improving safety. <P>SOLUTION: This is the lithium ion secondary battery equipped with (a) a positive electrode composed of a complex lithium oxide, (b) a negative electrode composed of a material capable of electrochemically storing and releasing lithium, (c) a separator, (d) a nonaqueous electrolytic solution, and (e) a porous film formed on either one surface of the positive electrode and the negative electrode. This is the lithium ion secondary battery in which that porous film is composed of inorganic oxide particles and a binder, and on the surface side and the electrode side of the porous film (side contacted with positive electrode and negative electrode), particle diameters of the inorganic oxide particles constituting the porous film are respectively different, and the particle diameters of the inorganic oxide particles of the surface side of the porous film are larger than those of the inorganic oxide particles of the electrode side of the porous film. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a non-aqueous electrolyte secondary battery excellent in safety such as short circuit resistance and heat resistance, and particularly relates to a lithium ion secondary battery.

  In a chemical battery such as a lithium ion secondary battery, a separator having a function of electrically insulating each electrode plate is disposed between a positive electrode and a negative electrode. As the separator, a microporous film mainly made of polyolefin resin is used.

  However, the separator made of the polyolefin resin has a heat resistance temperature of about 120 to 160 ° C., and when the battery is short-circuited internally or when a sharp projection such as a nail penetrates the battery, Has a problem that the battery is in a high temperature state.

Therefore, as a method for solving the above problems, a method of forming a porous film made of an inorganic powder such as alumina and a resin binder on the surface of either the positive electrode plate or the negative electrode plate has been proposed.
Japanese Patent Laid-Open No. 07-220759

  However, when a battery is prepared using an electrode plate on which a porous film is formed based on the prior art, the discharge characteristics of the battery, in particular, the discharge characteristics during low temperature environments and large current discharges are significantly reduced and storage characteristics. As a result, there was a problem that the cycle characteristics deteriorated.

  The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a lithium ion secondary battery that achieves both high safety, good discharge characteristics, and cycle characteristics by improving the film structure of the porous film.

(A) a positive electrode comprising a composite lithium oxide;
(B) a negative electrode,
(C) separator,
(D) a non-aqueous electrolyte, and (e) a lithium ion secondary battery comprising a porous film formed on the surface of any one of the positive electrode and the negative electrode, the porous film comprising inorganic oxide particles and The particle diameter of the inorganic oxide particles constituting the porous film is different between the surface side of the porous film and the side in contact with the positive electrode or the negative electrode (electrode side). The lithium ion secondary battery is characterized in that the particle diameter of the inorganic oxide particles on the side is larger than the particle diameter of the inorganic oxide particles on the electrode side of the porous membrane.

  Furthermore, in the porous film, the particle diameter of the inorganic oxide particles forming the surface side of the porous film is 1 μm or more and 3 μm or less, and the particle diameter of the inorganic oxide particles forming the electrode side of the porous film is 0.00. It is a lithium ion secondary battery characterized by being 1 micrometer or more and 0.5 micrometer or less.

As described above, according to the present invention, for the purpose of improving the internal short circuit safety, the lithium ion secondary battery in which a heat-resistant porous film made of an inorganic powder is formed on the electrode plate surface is excellent. It is possible to provide a battery having excellent discharge characteristics and high safety.

  Preferred embodiments of the present invention are shown below.

The present invention
(A) a positive electrode comprising a composite lithium oxide;
(B) a negative electrode,
(C) separator,
(D) a non-aqueous electrolyte, and (e) a lithium ion secondary battery comprising a porous film formed on the surface of any one of the positive electrode and the negative electrode, the porous film comprising inorganic oxide particles and The particle size of the inorganic oxide particles on the surface side of the porous film is different from that of the inorganic oxide particles constituting the porous film on the surface side of the porous film and the electrode side. The lithium ion secondary battery is characterized in that the diameter is larger than the particle diameter of the inorganic oxide particles on the electrode side of the porous membrane.

  On the surface side of the porous film, the pore diameter of the porous film can be increased by increasing the particle diameter of the inorganic oxide particles forming the porous film, thereby improving the discharge characteristics at a large current. . On the other hand, on the electrode side of the porous film, the voids of the porous film can be increased by reducing the particle diameter of the inorganic oxide particles forming the porous film, and thus the electrolyte solution between the porous film and the electrode can be increased. Retention is improved and cycle characteristics are improved.

  When the porous membrane is composed of inorganic oxide particles having a particle size of 0.1 μm or more and 0.5 μm or less, the pore size of the porous membrane is in the range of 0.07 to 0.10 μm, and the porosity is A film with a range of 50 to 60% is obtained. Since such a porous film has a large porosity, the amount of electrolyte solution retained is large, and by being constructed on the electrode side, the electrolyte solution retention at the electrode is increased and the cycle characteristics are improved. On the other hand, when the porous membrane is composed of inorganic oxide particles having a particle size of 1 μm or more and 3 μm or less, the pore size of the porous membrane is in the range of 0.4 to 0.5 μm, and the porosity is 25 to 35%. A film with a range is obtained. Since such a porous film has a large pore diameter, the discharge characteristics at a large current are improved. However, due to its low porosity, cycle characteristics are degraded. Therefore, a battery having excellent discharge characteristics and cycle characteristics at a large current can be obtained by optimally disposing the two kinds of porous films having different pore diameters and porosity. That is, in the porous film, the particle diameter of the inorganic oxide particles forming the surface side of the porous film is 1 μm or more and 3 μm or less, and the particle diameter of the inorganic oxide particles forming the electrode side of the porous film is 0.1 μm or more. Most preferably, it is 0.5 μm or less.

  As a manufacturing method for forming a porous film on the positive electrode or the negative electrode, a porous film precursor containing inorganic oxide particles, a binder, and a solvent is applied to and dried on either the positive electrode or the negative electrode. When forming a film, the porous film is applied first and repeatedly using two or more kinds of porous film precursors having different inorganic oxide particle diameters as the porous film precursor, and the porous film is first applied. It can manufacture by making the particle diameter of the inorganic oxide particle of the porous membrane precursor apply | coated later larger than the particle diameter of the inorganic oxide particle of a precursor.

The inorganic oxide particles forming the porous film are required to be electrically insulating and heat resistant. That is, even when the battery generates heat due to an internal short circuit due to some external factor, the heat resistance function of the inorganic oxide particles forming the porous film prevents the battery from generating heat and keeps the battery safe. Because you can. Therefore, the inorganic oxide particles constituting the porous film are electrically insulating and require heat resistance of 250 ° C. or higher. Moreover, it is necessary to be electrochemically stable within the potential window of the lithium ion secondary battery. As materials satisfying these conditions, materials such as alumina, silica, zirconia, and titania are more preferable.

  The amount of the resinous polymer binder used for the porous membrane is preferably 1 part by weight or more and 20 parts by weight or less with respect to 100 parts by weight of the inorganic oxide particles. When the added amount of the binder exceeds 20 parts by weight, there is a problem that the pores of the porous film are blocked with the binder and the discharge characteristics are deteriorated, and the added amount of the binder is less than 1 part by weight. In this case, the adhesion between the porous film and the electrode plate is lowered, so that the removal of the porous film becomes a problem.

  Examples of the binder for forming the porous film include styrene butadiene rubber (hereinafter abbreviated as SBR), polytetrafluoroethylene (hereinafter abbreviated as PTFE), polyvinylidene fluoride (hereinafter abbreviated as PVDF), and acrylic acid of SBR. Although a modified body etc. can be used, Furthermore, the binder of the porous film of preferable embodiment of this invention is a thing whose melting | fusing point or decomposition start temperature is 250 degreeC or more. Furthermore, the binder for the porous membrane according to a preferred embodiment of the present invention includes a rubbery polymer containing an acrylonitrile unit. When an internal short circuit occurs, the heat generation temperature of the short circuit part is about 100 ° C. When the melting point is low or the thermal decomposition temperature is low, the binder softens or burns out. As a result, the porous film is deformed, and the short-circuit portion is further expanded. This is because such a problem must be avoided.

  In the lithium ion secondary battery of a further preferred embodiment of the present invention, the positive electrode and the negative electrode are laminated via a separator, and they are wound in a spiral shape.

  The thickness of the porous film is not particularly limited, but is preferably 0.5 to 20 μm from the viewpoint of sufficiently exerting the function of improving the safety by the porous film and maintaining the design capacity of the battery. Even when two or more porous films are formed, the total thickness is preferably 0.5 to 20 μm. In this case, the sum of the thickness of the separator and the thickness of the porous film that are generally used at present is preferably 15 to 30 μm.

  The positive electrode includes at least a positive electrode active material, a binder, and a conductive agent.

  An example of the positive electrode active material is a composite oxide. As the composite oxide, lithium cobaltate, modified lithium cobaltate, lithium nickelate, modified lithium nickelate, lithium manganate, modified lithium manganate, and the like are preferable. Some modified bodies contain elements such as aluminum and magnesium. There are also those containing at least two of cobalt, nickel and manganese.

  The binder used for the positive electrode is not particularly limited, and PTFE, modified acrylonitrile rubber particles, PVDF, and the like can be used. PTFE or the like is preferably used in combination with carboxymethyl cellulose, polyethylene oxide, modified acrylonitrile rubber, or the like that serves as a thickener for the raw material paste of the positive electrode mixture layer. PVDF is single and functions as both a binder and a thickener.

  As the conductive agent, acetylene black, ketjen black (registered trademark), various graphites, and the like can be used. These may be used alone or in combination of two or more.

  The negative electrode includes at least a negative electrode active material and a binder.

As the negative electrode active material, various natural graphites, various artificial graphites, silicon-containing composite materials such as silicide, and various alloy materials can be used. As the binder, various binders such as PVDF and modified products thereof can be used.

Various lithium salts such as LiPF ₆ and LiBF ₄ can be used as solutes in the electrolyte solution composed of a non-aqueous solvent. As the non-aqueous solvent, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate and the like are preferably used, but are not limited thereto. Although a nonaqueous solvent can also be used individually by 1 type, it is preferable to use 2 or more types in combination. Moreover, as an additive, vinylene carbonate, cyclohexylbenzene, those modified bodies, etc. can also be used.

  The separator is not particularly limited as long as it is made of a material that can withstand the use environment of the lithium ion secondary battery, but a microporous film made of a polyolefin-based resin such as polyethylene or polypropylene is generally used. The microporous film may be a single layer film made of one kind of polyolefin resin or a multilayer film made of two or more kinds of polyolefin resin.

(Example)
3 kg of lithium cobaltate was stirred in a double-arm kneader together 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. A positive electrode paste was prepared. This paste was applied and dried on both sides of a 15 μm thick aluminum foil, rolled to a total thickness of 160 μm, and then cut and lead-attached into a width of 60 mm and a length of 500 mm to obtain a positive electrode plate.

  2 kg of artificial graphite was stirred in a double-arm kneader together with 75 g of styrene butadiene rubber binder BM-400B (solid content 40 wt%), 30 g of carboxymethylcellulose (CMC) and an appropriate amount of water manufactured by Nippon Zeon Co., Ltd. A negative electrode paste was prepared. This paste was applied and dried on both sides of a 10 μm thick copper foil and rolled to a total thickness of 180 μm to obtain a negative electrode reel.

  Next, using alumina powders having particle sizes of 0.1, 0.2, 0.5, 1.0, 2.0, and 3.0 μm, respectively, 950 g of the alumina powder was added to Nippon Zeon ( Co., Ltd. polyacrylonitrile modified rubber binder BM-720H (solid content 8% by weight) 625 g and an appropriate amount of NMP and stirred in a double-arm kneader to prepare 7 types of porous film pastes, A, B, C , D, E, F were prepared.

  Next, the porous film pastes A, B, C, D, and E are applied and dried so as to have a thickness of 3 μm on both surfaces of the negative electrode reel, and five types of negative electrode reels a, b, c, d, and e are obtained. Obtained. Next, porous film pastes B, C, D, E, and F were applied and dried so as to have a thickness of 3 μm on both surfaces of the five types of negative electrode reels, respectively, to prepare a two-layer porous film. At this time, the particle diameter of the alumina powder to be applied later to form the porous film on the surface side is combined to be larger than the particle diameter of the alumina previously applied to form the porous film on the electrode side. A porous membrane was produced. In this way, 15 types of negative electrode reels AB, AC, AD, AE, AF, BC, BD, BE, BF, CD, CE, CF, DE, DF, and EF were obtained (here, the types of the negative electrodes (The symbol of the porous film paste formed on the electrode side) (the symbol of the porous film paste on the surface side) is used in combination. The obtained 15 types of negative electrode reels were cut and lead-attached into dimensions of a width of 62 mm and a length of 570 mm to obtain negative electrode plates, respectively.

  As the separator, a polyethylene microporous film (film thickness: 16 μm) was used.

A positive electrode plate and a negative electrode plate are wound together with a separator, and the wound one is inserted into a cylindrical battery case having a diameter of 18 mm and a height of 67 mm, and after processing such as welding of a sealing plate, the electrolyte is 1 Using 0.0 M-LiPF ₆ / EC + EMC (volume ratio 1: 3), 5.5 g of the electrolyte was injected to produce a cylindrical battery having a diameter of 18 mm and a height of 65 mm. The design capacity of this battery is 2000 mAh. This is an example.
(Comparative example)
950 g of the alumina powder was used by using powders of alumina powder having particle sizes of 0.1, 0.2, 0.5, 1.0, 2.0, and 3.0 μm, respectively. Nippon Zeon Co., Ltd. 7 types of porous film pastes, A, B, C, and D, which were stirred together with a 625 g polyacrylonitrile-modified rubber binder BM-720H (solid content 8 wt%) and an appropriate amount of NMP in a double-arm kneader. , E, F were prepared.

  Next, porous film pastes A, B, C, D, E, and F were applied and dried so as to have a thickness of 3 μm on both sides of the negative electrode reel obtained by the same method as in the example, and six types of negative electrode reels a , B, c, d, e, f were obtained. Next, the porous film pastes A, B, C, D, E, and F were applied and dried so as to have a thickness of 3 μm on both surfaces of the six types of negative electrode reels, respectively. 21 types of negative electrode reels AA, BA, BB, CA, CB, CC, DA, DB, DC, DD, EA, EB, EC, ED, EE, FA, FB, FC, FD, FE, FF were obtained.

  The obtained 21 types of negative electrode reels were cut and lead-attached into dimensions of a width of 62 mm and a length of 570 mm to obtain negative electrode plates, respectively.

  A cylindrical battery was produced in the same manner as in the example except that the above negative electrode plate was used, and used as a comparative example.

The batteries of the above examples and comparative examples were evaluated by the following method.
(Large current discharge test)
The low temperature discharge test was conducted by the following method. The battery was charged at a constant temperature of 20 ° C. under a charging voltage of 4.2 V and a charging maximum current of 1400 mA for 2 hours, followed by a constant current discharging with a discharging current of 200 mA and a discharge end voltage of 3.0 V. The capacity was measured and used as the rated capacity of the battery. Next, after charging the discharged battery again in the same manner as described above, the charged battery was discharged at a constant current of 4000 mA and an end-of-discharge voltage of 3.0 V at an environmental temperature of 20 ° C. The discharge capacity of was measured. The ratio of the discharge capacity at the time of large current discharge to the rated capacity of the battery was determined and used as the discharge capacity maintenance rate.
(Cycle test)
The charge / discharge cycle life test was conducted by the following method. After charging at a constant voltage for 2 hours at an environmental temperature of 20 ° C. under conditions of a charging voltage of 4.2 V and a charging maximum current of 1400 mA, a constant current discharging with a discharging current of 2000 mA and a discharge final voltage of 3.0 V was performed. The charge and discharge cycles were repeated, and the ratio of the discharge capacity at the 500th cycle to the discharge capacity at the first cycle was determined as the cycle capacity maintenance rate.

  In the following, the results will be described in detail with respect to the present invention.

  Table 1 shows the relationship of combinations of porous films formed on the negative electrode plates of the examples and comparative examples. Next, Table 2 shows the results of the discharge capacity retention ratio during large current discharge.

  As shown in Table 2, the negative electrode, AD, AE, AF, BD, BE, BF, CD when the particle size of the alumina powder forming the surface side of the porous membrane is 1.0, 2.0, 3.0 μm , CE, CF, DD, DE, DF, ED, EE, EF, FD, FE, and FF all showed good characteristics with a discharge capacity retention rate of 95% or more. This is because, when a porous film is formed using particles having a particle diameter of 1 μm or more and 3 μm or less, the pore diameter of the porous film is in the range of 0.4 to 0.5 μm, and the porosity is in the range of 25 to 35%. A film is obtained. Such a porous film has a large pore diameter, and is considered to improve discharge characteristics at a large current.

  On the other hand, the negative electrode, AA, AB, AC, BA, BB, BC, CA, CB, CC, when the particle diameter of the alumina powder forming the surface side of the porous film is 0.1, 0.2, 0.5 μm, DA, DB, DC, EA, EB, EC, FA, FB, and FC all show low discharge characteristics of 90% or less, and discharge characteristics as the particle size of the alumina powder forming the surface side decreases. Decreased. This is because, when a porous membrane is formed using particles having a particle size of 0.1 μm or more and 0.5 μm or less, the pore size of the porous membrane is in the range of 0.07 to 0.10 μm, and the porosity is from 50 A film with a range of 60% is obtained. Such a porous film is considered to have a reduced discharge characteristic at a large current due to its small pore diameter.

  Table 3 shows the results of the cycle test.

  As shown in Table 3, the negative electrode, AA, AB, AC, AD, AE, AF, BA when the particle diameter of the alumina powder forming the electrode side of the porous membrane is 0.1, 0.2, 0.5 μm , BB, BC, BD, BE, BF, CA, CB, CC, CD, CE, and CF all showed good cycle characteristics with a cycle capacity retention rate of 85% or more. This is because, when a porous membrane is formed using particles having a particle size of 0.1 μm or more and 0.5 μm or less, the pore size of the porous membrane is in the range of 0.07 to 0.10 μm, and the porosity is from 50 A film with a range of 60% is obtained. Since such a porous membrane has a large porosity, it retains a large amount of electrolyte solution. By constructing such a porous membrane on the electrode side, the retention property of the electrolyte solution at the electrode is increased and the cycle characteristics are improved. It is considered a thing.

  On the other hand, the negative electrode, DA, DB, DC, DD, DE, DF, EA, EB, EC, when the particle diameter of the alumina powder forming the electrode side of the porous membrane is 1.0, 2.0, 3.0 μm ED, EE, EF, FA, FB, FC, FD, FE, and FF all exhibited low cycle characteristics with a cycle capacity retention rate of 80% or less. This is because, when a porous film is formed using particles having a particle diameter of 1 μm or more and 3 μm or less, the pore diameter of the porous film is in the range of 0.4 to 0.5 μm, and the porosity is in the range of 25 to 35%. A film is obtained. Since such a porous membrane has a low porosity, it is considered that the retention of the electrolytic solution is lowered and the cycle characteristics are lowered.

  From the above results, it can be seen that a battery excellent in discharge characteristics and cycle characteristics at a large current can be obtained by optimally arranging two kinds of porous films having different pore diameters and porosity as in the present invention.

  In the negative electrode of AB, AC, BC, the particle diameter of the inorganic oxide particles forming the surface side of the porous film is larger than the particle diameter of the inorganic oxide particles forming the electrode side of the porous film. Since the particle diameter of the inorganic oxide particles forming the surface side of the porous film is 0.2 μm or 0.5 μm, the pore diameter of the porous film is reduced, and the large current discharge characteristics are deteriorated. Further, in the negative electrode of DE, DF, and EF, the particle diameter of the inorganic oxide particles forming the surface side of the porous film is larger than the particle diameter of the inorganic oxide particles forming the electrode side of the porous film. However, since the particle diameter of the inorganic oxide particles forming the electrode side of the porous film is 1.0 μm or 2.0 μm, the porosity of the porous film is reduced, and the retention of the electrolytic solution at the electrode is reduced. The cycle characteristics have deteriorated.

From the above results, in order to obtain a battery having excellent discharge characteristics at a large current and excellent cycle characteristics, the particle diameter of the inorganic oxide particles forming the surface side of the porous film is more porous. Forming a porous film larger than the particle diameter of the inorganic oxide particles forming the side, and setting the particle diameter of the inorganic oxide particles forming the surface side of the porous film to 1 μm to 3 μm, It is desirable that the inorganic oxide particles to be formed have a particle size of 0.1 μm or more and 0.5 μm or less.

  That is, in the porous film, the pore diameter of the porous film can be increased by setting the particle diameter of the inorganic oxide particles forming the surface side of the porous film to 1 μm or more and 3 μm or less, thereby improving the discharge characteristics at a large current. it can. Moreover, the porosity of the porous membrane is increased by setting the particle size of the inorganic oxide particles forming the electrode side of the porous membrane to 0.1 μm or more and 0.5 μm or less, and the electrolyte retention on the electrode side is increased. The cycle characteristics can be improved.

  In the examples, two types of inorganic oxide particles having different particle sizes were used to form a two-layer porous film. However, the particle size of the inorganic oxide particles forming the surface side of the porous film was described. Forming a porous film larger than the particle diameter of the inorganic oxide particles forming the electrode side of the porous film, and the particle diameter of the inorganic oxide particles forming the surface side of the porous film being 1 μm or more and 3 μm or less, If the particle diameter of the inorganic oxide particles forming the electrode side of the porous membrane is 0.1 μm or more and 0.5 μm or less, the same effect can be obtained not only with two layers but also with a plurality of layers.

Thus, according to the present invention, it is clear that a battery having excellent discharge characteristics at a large current and excellent cycle characteristics can be obtained.

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

Claims (4)

(A) a positive electrode comprising a composite lithium oxide;
(B) a negative electrode,
(C) separator,
(D) a non-aqueous electrolyte, and (e) a lithium ion secondary battery comprising a porous film formed on the surface of one of the positive electrode and the negative electrode,
The porous film is composed of inorganic oxide particles and a binder, and the particle diameter of the inorganic oxide particles constituting the porous film is on the surface side of the porous film and the side in contact with the positive electrode or the negative electrode (electrode side). A lithium ion secondary battery, which is different from each other and wherein the particle diameter of the inorganic oxide particles on the surface side of the porous film is larger than the particle diameter of the inorganic oxide particles on the electrode side of the porous film.   The particle diameter of the inorganic oxide particles forming the surface side of the porous film is 1 μm or more and 3 μm or less, and the particle diameter of the inorganic oxide particles forming the electrode side of the porous film is 0.1 μm or more and 0.5 μm or less. The lithium ion secondary battery according to claim 1, wherein:   About the manufacturing method of the lithium ion secondary battery which forms a porous film in an electrode, the porous film precursor containing an inorganic oxide particle, a binder, and a solvent is applied and dried on the surface of either the positive electrode or the negative electrode. When forming a porous film, the porous film precursor is a porous film having a multilayer structure that is repeatedly applied and dried using two or more kinds of porous film precursors having different particle diameters of inorganic oxide particles, and The particle diameter of the inorganic oxide particles of the porous film precursor applied later is larger than the particle diameter of the inorganic oxide particles of the porous film precursor applied first, and finally the porous film precursor applied to the surface 2. The method for producing a lithium ion secondary battery according to claim 1, wherein the inorganic oxide particles have a maximum particle size. The particle diameter of the inorganic oxide particles of the porous film precursor applied first is 0.1 μm or more and 0.5 μm or less, and the particle diameter of the inorganic oxide particles of the porous film precursor applied last on the surface The method for producing a lithium ion secondary battery according to claim 3, wherein the thickness is set to 1 μm or more and 3 μm or less.