JP2014199714A - Negative electrode for nonaqueous electrolyte secondary battery and nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve such a problem that, when an internal short circuit due to such as sticking of a foreign object like a nail occurs, great stress is applied on a negative electrode even when a layer assuming insulation properties is arranged on a surface of a main active material layer, so that adhesion between a collector and the main active material layer becomes insufficient, the collector is easily exposed due to the separation of the main active material layer together with the layer assuming insulation properties from a collector surface, and thus it is difficult to secure insulation properties.SOLUTION: A nonaqueous electrolyte secondary battery uses a negative electrode in which a layer assuming insulation properties is formed to have slight conductivity and disposed on a side of a collector surface, as a high resistance layer.

Description

The present invention relates to a negative electrode for a nonaqueous electrolyte secondary battery, and more particularly to a negative electrode for a nonaqueous electrolyte secondary battery with improved safety.

  In recent years, there has been a demand for higher energy density for secondary batteries used in electronic devices such as mobile phones and laptop computers and in-vehicle power supplies. From this viewpoint, non-aqueous electrolyte secondary batteries that can increase energy density are widely used. It is popular. The non-aqueous electrolyte secondary battery includes a positive electrode, a negative electrode, a separator interposed therebetween, and a non-aqueous electrolyte. Many of the positive electrode, the negative electrode, and the separator are wound to form an electrode group.

  In general, when a short circuit having a relatively low resistance value occurs inside the battery, a large current flows in a concentrated manner at the short circuit point, so that the heat generation of the battery may accelerate and lead to overheating. In order to avoid such a phenomenon in a lithium ion battery having a high energy density, various safety measures are taken from the viewpoint of the battery configuration in addition to the viewpoint of manufacturing.

  In general, a separator having a shutdown function for blocking pores and blocking ionic current due to heat generated when a battery causes an internal short circuit is used. The short-circuit current stops flowing due to the shutdown function, and the heat generation stops. However, when the heat generation in the short-circuited portion is large, the separator is melted before the shutdown functions, thereby causing a melt-down that opens a large hole in the separator. When the positive electrode and the negative electrode are short-circuited due to meltdown, further overheating is caused. In some cases, the battery ignites or smokes, which is very dangerous.

  Therefore, a technique for forming a porous film made of an inorganic oxide filler such as alumina and a water-soluble polymer on an active material layer has been proposed (see Patent Document 1). If the technique of patent document 1 is used, it is thought that the capability to maintain the insulation between positive and negative electrodes is high also in an overheating environment.

  Furthermore, a technique for forming a surface layer made of a lithium titanium composite oxide capable of inserting and extracting lithium on the main negative electrode layer has been proposed (see Patent Document 2).

JP-A-9-147916 JP 2010-97720 A

  However, in the conventional technology, a layer responsible for insulation at the time of a short circuit is arranged on the surface side, and a main active material layer is arranged on the current collector side. In the event of an internal short circuit that involves destruction of the electrode plate due to a large foreign object, the insulating layer may be peeled off from the current collector together with the underlying main active material layer, resulting in insufficient insulation effects. .

In other words, in the case of an internal short circuit where a foreign object such as a nail is pierced, even if a layer responsible for insulation is disposed on the surface of the main active material layer, a large stress is applied to the negative electrode. Adhesion with the material layer becomes insufficient, and the main active material layer and the insulating layer are peeled together from the surface of the current collector, so that the current collector is easily exposed. Therefore, it is difficult to ensure insulation. I understood. However, if the amount of the binder in the main active material layer is increased so that sufficient adhesion between the current collector and the main active material layer can be secured, the lithium ion acceptability deteriorates and the battery characteristics are remarkably increased. descend.

  An object of the present invention is to provide a negative electrode excellent in safety capable of suppressing an excessive increase in battery temperature even when a large foreign object such as a nail is stuck and an internal short circuit occurs, and a battery including the negative electrode. It is.

  As a result of intensive studies by the present inventors, the layer responsible for insulation has a slight conductivity, and when placed on the current collector surface side as a high resistance layer, an appropriate amount of binder in the high resistance layer As a result, it became clear that even when a large stress is applied to the negative electrode, the high resistance layer is difficult to peel from the current collector, and as a result, the high resistance is maintained and the safety is excellent.

One aspect of the present invention includes a current collector, a first layer formed on a surface of the current collector, and a second layer stacked on the first layer, and the first layer includes an inorganic solid. It includes oxide particles, wherein the second layer comprises a carbon material, wherein the thickness of the first layer and T _1, the thickness of the second layer when the T _2, characterized in that T ₂ is greater than T ₁ And a negative electrode for a non-aqueous electrolyte secondary battery.

  Another aspect of the present invention relates to a nonaqueous electrolyte secondary battery including the negative electrode.

  The negative electrode for a non-aqueous electrolyte secondary battery according to the present invention has a first layer containing inorganic solid oxide particles as a current collector even when an internal short circuit accompanied by destruction of the electrode plate by a large foreign object such as a nail occurs. Easy to maintain binding. As a result, high resistance between the positive and negative electrodes can be easily maintained, and an excessive increase in battery temperature can be suppressed.

Sectional drawing of the negative electrode for lithium ion batteries which concerns on one Embodiment of this invention. 1 is a longitudinal sectional view schematically showing a configuration of a nonaqueous electrolyte secondary battery according to an embodiment of the present invention.

In FIG. 1, the longitudinal cross-sectional conceptual diagram of the negative electrode for nonaqueous electrolyte secondary batteries which concerns on one Embodiment of this invention is shown. The negative electrode 6 includes a first layer 12 formed on the surface of the negative electrode current collector 11 and a second layer 13 laminated on the first layer 12, and the first layer 12 includes inorganic solid oxide particles, the two layers 13 include a carbon material, the thickness of the first layer 12 and _{T 1,} when the thickness of the second layer 13, _{T 2, T 2} is greater than _{T 1.}

As the inorganic solid oxide particles contained in the first layer 12, insulating oxides that do not occlude / release lithium ions, such as alumina, silica, and magnesia, can be used. Particularly preferred is lithium titanate having a crystal structure. Lithium titanate has high acceptability of lithium ions, and it is easy to reduce the diffusion resistance of the electrode. Furthermore, lithium titanate does not have conductivity in a state where lithium ions are not occluded, and has higher thermal stability than a carbon material. Therefore, when an internal short-circuit occurs, the lithium ions that have been occluded along with the discharge are released to lose conductivity and function as an insulating layer, so that heat generation is also suppressed. Lithium titanate having a typical spinel crystal structure is represented by the formula: Li ₄ Ti ₅ O ₁₂ . However, the general _{formula: Li x Ti 5-y M} y O 12 + lithium titanate represented by _z may be used as well. Here, M is composed of vanadium, manganese, iron, cobalt, nickel, copper, zinc, aluminum, boron, magnesium, calcium, strontium, barium, zirconium, niobium, molybdenum, tungsten, bismuth, sodium, gallium, and rare earth elements. At least one selected from the group, x is the value of lithium titanate immediately after synthesis or in a fully discharged state, 3 ≦ x ≦ 5, 0.005 ≦ y ≦ 1.5, − 1 ≦ z ≦ 1.

  The first layer 12 preferably contains a binder and a conductive material in addition to the inorganic solid oxide particles. Examples of the binder include PVDF, PTFE, polyethylene, polypropylene, aramid resin, polyamide, polyimide, polyamideimide, polyacrylonitrile, polyacrylic acid, polyacrylic acid methyl ester, polyacrylic acid ethyl ester, polyacrylic acid hexyl. Esters, polymethacrylic acid, polymethacrylic acid methyl ester, polymethacrylic acid ethyl ester, polymethacrylic acid hexyl ester, polyvinyl acetate, polyvinylpyrrolidone, polyether, polyethersulfone, hexafluoropolypropylene, styrene butadiene rubber or carboxymethyl cellulose Can be used. Examples of the binder include tetrafluoroethylene, hexafluoroethylene, hexafluoropropylene, perfluoroalkyl vinyl ether, vinylidene fluoride, chlorotrifluoroethylene, ethylene, propylene, pentafluoropropylene, fluoromethyl vinyl ether, and acrylic acid. Alternatively, a copolymer of two or more materials selected from hexadiene may be used. Moreover, you may mix and use 2 or more types selected from the said material for a binder, for example. The amount of the binder is not particularly limited as long as the adhesion between the first layer 12 and the current collector can be secured, but is preferably 0.5 to 10 parts by weight per 100 parts by weight of the inorganic solid oxide particles. The conductive material may be, for example, natural graphite or artificial graphite, carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black or thermal black, carbon Conductive fibers such as fibers or metal fibers may be used, and may be carbon fluoride. Although the amount of the conductive material is not particularly limited, the conductive material functions as a resistance layer when a short circuit occurs due to a foreign matter such as a nail while maintaining electrical connection between the second layer 13 and the current collector. 1 to 5 parts by weight is preferable.

  Examples of the carbon material included in the second layer 13 include carbon materials such as various natural graphite, coke, graphitized carbon, carbon fiber, spherical carbon, various artificial graphite, and amorphous carbon. The second layer 13 contains a simple substance such as silicon (Si) or tin (Sn), a silicon compound (for example, a silicon alloy, a solid solution containing silicon) or a tin compound (for example, a tin alloy or a solid solution containing tin). You may go out. The second layer 13 preferably contains a binder. The second layer 13 may include a conductive material. Although the lamination | stacking method of the 1st layer 12 and the 2nd layer 13 is not specifically limited, For example, after forming the 1st layer 12 by application | coating and drying, it may laminate | stack on the 1st layer 12 surface, and may be applied and dried. Alternatively, the coating material of the first layer 12 and the coating material of the second layer 13 may be applied simultaneously using a die coater having two or more discharge slits, and then dried.

The thickness T ₁ of the first layer 12 becomes too thick relative to the thickness T ₂ of the second layer 13, with the energy density of the whole negative electrode is lowered, the binder in the first layer 12 is the surface layer during drying The ratio T ₁ / T ₂ with the thickness T ₂ of the second layer 13 needs to be smaller than 1 in order to migrate to the side and impair the adhesion with the current collector. The thickness T ₁ of the first layer 12 is preferably 1 ≦ T ₁ ≦ 30 μm. If T ₁ is smaller than ₁ μm, high resistance at the time of short circuit may be impaired, and if T ₁ is larger than 30 μm, it is not preferable in terms of adhesion to the current collector and energy density.

  The negative electrode current collector 11 is preferably a copper foil, a copper alloy foil or a nickel foil. Although the thickness of a collector is 5-30 micrometers, for example, it is not specifically limited.

The positive electrode combined with the negative electrode of the present invention usually comprises a positive electrode current collector and a positive electrode mixture layer carried on the positive electrode current collector. The positive electrode mixture layer only needs to contain a binder and a conductive agent in addition to the positive electrode active material. A lithium composite metal oxide can be used as the positive electrode active material. Examples of the lithium composite metal oxide include Li _x CoO ₂ , Li _x NiO ₂ , Li _x MnO ₂ , Li _x Co _y Ni _1-y O ₂ , Li _x Co _y M _1-y O _z , and Li _x Ni _{1. -y} M _y O
_{z, Li x Mn 2 O 4} , Li x Mn 2-y M y O 4, LiMPO 4, Li 2 MPO 4 F (M = Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn , Al, Cr, Pb, Sb and B). Here, 0 <x ≦ 1.2, 0 <y ≦ 0.9, and 2.0 ≦ z ≦ 2.3. In addition, x value which shows the molar ratio of lithium is a value immediately after active material preparation, and increases / decreases by charging / discharging. In addition, the positive electrode active material may be one in which a part of the metal element in the lithium composite metal oxide is replaced with a different element. Furthermore, the positive electrode active material may be one in which the lithium composite metal oxide is surface-treated with a metal oxide, lithium oxide, or a conductive agent, or the surface of the lithium composite metal oxide is hydrophobized. It may be what was done.

The electrolyte layer having lithium ion conductivity includes a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The electrolyte layer may include a polyolefin microporous film as a separator. In this case, a nonaqueous solvent in which a lithium salt is dissolved is impregnated in the pores of the microporous film. Examples of the non-aqueous solvent include ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). It is not limited to. These may be used alone or in combination of two or more. Examples of the lithium salt include LiBF ₄ , LiPF ₆ , LiAlCl ₄ , LiCl, and lithium imide salt. These may be used alone or in combination of two or more.

  Hereinafter, the present invention will be described in detail based on examples.

<Example 1>
(I) Production of negative electrode (first layer)
2 kg of lithium titanate having a spinel crystal structure (Li ₄ Ti ₅ O ₁₂ , average particle size 1 μm, BET specific surface area 3 m ² / g), artificial graphite (average particle size 10 μm) 50 g, manufactured by Nippon Zeon Co., Ltd. BM-400B (modified styrene-butadiene rubber dispersion having a solid content of 40% by weight) and 50 g of carboxymethylcellulose (CMC) were stirred together with an appropriate amount of water in a double-arm kneader, and titanic acid. A first negative electrode mixture slurry containing lithium was prepared. The first negative electrode mixture slurry was applied to both surfaces of a negative electrode current collector made of a copper foil having a thickness of 10 μm, dried, and rolled to a total thickness of 30 μm to form a first layer. That is, the thickness (T ₁ ) of the first layer was 10 μm per side of the copper foil, and the density of the first layer was 2 g / cm ³ .

(Second layer)
3 kg of artificial graphite (average particle size 10 μm, BET specific surface area 3 m ² / g), 75 g of BM-400B (dispersion of modified styrene-butadiene rubber having a solid content of 40% by weight) manufactured by Nippon Zeon Co., Ltd., and carboxymethylcellulose (CMC) 30 g was stirred together with an appropriate amount of water in a double-arm kneader to prepare a second negative electrode mixture slurry containing graphite. The second negative electrode mixture slurry was applied to the surface of the first layer provided on both sides of the copper foil, dried, and rolled to a total thickness of 160 μm to form a second layer. That is, the thickness (T ₂ ) of the second layer was 65 μm per one side of the copper foil, and the density of the second layer was 1.5 g / cm ³ .

The obtained electrode plate was cut into a width of 58 mm and a length of 750 mm to obtain a negative electrode. This negative electrode satisfies T ₁ / T ₂ = 0.15.

(Ii) Production of Positive Electrode N-methyl-pyrrolidone (hereinafter referred to as NMP (N-methylpyrrolidon) containing 3 kg of lithium cobaltate (LiCoO ₂ , average particle size 10 μm) and 12% by weight of PVDF
e)) 1 kg of a solution (trade name: PVDF # 1320, manufactured by Kureha Co., Ltd.), 90 g of acetylene black and an appropriate amount of NMP were stirred with a double-arm kneader to prepare a positive electrode mixture slurry. The positive electrode mixture slurry was applied to both surfaces of a positive electrode current collector made of an aluminum foil having a thickness of 15 μm, dried, and rolled to a total thickness of 120 μm to form a positive electrode active material layer. The obtained electrode plate was cut into a width of 56 mm and a length of 700 mm to obtain a positive electrode.

(Nonaqueous electrolyte)
LiPF ₆ was dissolved at a concentration of 1 mol / liter in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 1: 1: 1. A non-aqueous electrolyte was obtained by adding 3% by weight of vinylene carbonate.

(Battery assembly)
A cylindrical battery as shown in FIG. 2 was produced.

  The positive electrode 5, the negative electrode 6, and the separator 7 were wound with the separator 7 (single layer of polyethylene resin having a thickness of 20 μm) interposed between the positive electrode 5 and the negative electrode 6 manufactured according to the above method. Thereby, the electrode group 9 was produced. After the upper insulating plate 8a and the lower insulating plate 8b were disposed at both ends of the electrode group 9 in the longitudinal direction, the electrode group 9 was housed in a bottomed cylindrical battery case 1 (diameter 18 mm, height 65 mm, inner diameter 17.85 mm). The other end of the positive electrode lead 5a made of aluminum was connected to the lower surface of the positive electrode terminal, and the other end of the negative electrode lead 6a made of nickel was connected to the inner bottom surface of the battery case 1. Thereafter, 5.5 g of the non-aqueous electrolyte described above was injected into the battery case 1. A sealing plate 2 that supports the positive electrode terminal was caulked to the opening of the battery case 1 through the gasket 3. Thereby, the battery case 1 was sealed. In this way, a cylindrical non-aqueous electrolyte secondary battery having a design capacity of 2150 mAh was produced. This battery is referred to as the battery of Example 1.

<Example 2>
A negative electrode was prepared in the same manner as in Example 1 except that the thickness T ₁ of the first layer and the thickness T ₂ of the second layer were 1 μm and 70 μm, respectively, and further a cylindrical nonaqueous electrolyte secondary battery was manufactured.

<Example 3>
A negative electrode was prepared in the same manner as in Example 1 except that the thickness T ₁ of the first layer and the thickness T ₂ of the second layer were 5 μm and 70 μm, respectively, and a cylindrical nonaqueous electrolyte secondary battery was also prepared.

<Example 4>
Cylindrical non-aqueous solution as in Example 1 except that the first layer thickness T ₁ and the second layer thickness T ₂ were 30 μm and 55 μm, respectively, the negative electrode length was 710 mm, and the positive electrode length was 660 mm. An electrolyte secondary battery was produced.

<Example 5>
Cylindrical non-aqueous electrolyte 2 was prepared in the same manner as in Example 1, except that the first layer thickness T ₁ and the second layer thickness T ₂ were 40 μm and 50 μm, respectively, the negative electrode length was 700 mm, and the positive electrode length was 650 mm. A secondary battery was produced.

<Example 6>
A negative electrode was prepared in the same manner as in Example 2 except that the inorganic solid oxide particles contained in the first layer were alumina (Al ₂ O ₃ , average particle size 0.3 μm), and further a cylindrical non-aqueous electrolyte secondary A battery was produced.

<Comparative Example 1>
Cylindrical non-aqueous electrolyte two as in Example 1, except that the first layer thickness T ₁ and the second layer thickness T ₂ were 50 μm and 40 μm, respectively, the negative electrode length was 680 mm, and the positive electrode length was 630 mm. A secondary battery was produced.

<Comparative example 2>
The thickness T ₂ of the thickness T ₁ and the second layer of the first layer, except that a 5μm and 70μm, respectively, to produce a cylindrical non-aqueous electrolyte secondary battery in the same manner as in Example 6.

<Comparative Example 3>
A cylindrical nonaqueous electrolyte secondary battery was produced in the same manner as in Example 2 except that the first layer was not formed.

(Battery evaluation method)
The batteries of Examples 1 to 6 and Comparative Examples 1 to 3 were subjected to the following nail penetration test and charge / discharge test to evaluate the safety of each battery and the battery characteristics of each battery.

[Nail penetration test]
The batteries of Examples 1 to 6 and Comparative Examples 1 to 3 were charged under the following conditions. Then, in a 20 ° C. environment, an iron nail having a diameter of 2.7 mm was pierced from the side surface of the charged battery to a depth of 2 mm at a speed of 5 mm / second to generate an internal short circuit. After 30 seconds from the nail penetration, the temperature of the battery was measured with a thermocouple placed on the side of the battery away from the nail penetration position. The results are shown in “Battery surface temperature” in Table 1.

-Charging conditions-
Constant current charging: Current value 0.5C, end-of-charge voltage 4.3V
Constant voltage charge: Voltage value 4.3V, charge end current 100mA
[Charge / discharge test]
The batteries of Examples 1 to 6 and Comparative Examples 1 to 3 were repeatedly charged and discharged 300 times under the following conditions in a 25 ° C. environment. The percentage (%) of the ratio of the final discharge capacity to the initial discharge capacity was determined and used as the capacity maintenance rate. The results are shown in “Capacity maintenance ratio” in Table 1.

−Charging / discharging conditions−
Constant current charging: Current value 1C, end-of-charge voltage 4.2V
Constant voltage charging: Voltage value 4.2V, charging end current 100mA
Constant current discharge: current value 1C, final discharge voltage 1.0V

  Hereinafter, the obtained results will be described in detail.

In Examples 1, 3, and 4, a battery having a low surface temperature and a high capacity retention rate when nailing was obtained. In Example 2, the surface temperature of the battery at the time of nail penetration was slightly high. This is probably because the first layer thickness T ₁ is as thin as ₁ μm, so that the high resistance at the time of short circuit is slightly lowered. In Examples 5 and 6, the capacity retention rate was slightly decreased. Example 5 is thick and the thickness T ₁ is 40μm of the first layer, for Example 6 using alumina is an insulating oxide which does not absorb and desorb lithium ions inorganic solid oxide particles in the first layer, respectively It is considered that the charge / discharge characteristics have deteriorated.

In Comparative Examples 1 and 2, the capacity retention rate was significantly reduced, and the battery capacity obtained was also small. Comparative Example 1 because the thickness T ₁ of the first layer is greater than the thickness T ₂ of the second layer, presumably because led to decrease in the reduction and energy density characteristics by acting as a resistance layer during charging and discharging. In Comparative Example 2, it is considered that since the resistance of the first layer containing alumina is high, the battery capacity cannot be obtained from the beginning, and the characteristics are deteriorated. In Comparative Example 3, since the first layer was not present, the battery surface temperature during nail penetration was high.

From the above results, the ratio T ₁ / T ₂ between the thickness T ₁ of the first layer and the thickness T ₂ of the second layer needs to be smaller than 1, and preferably 1 ≦ T ₁ ≦ 30 μm. I understood it.

  The present invention can provide a nonaqueous electrolyte secondary battery having excellent safety while preventing deterioration of battery characteristics. Therefore, the present invention is useful as a technology that can be applied not only to a small power source such as a portable electronic device but also to a large power source such as an EV (Electric Vehicle).

DESCRIPTION OF SYMBOLS 1 Battery case 2 Sealing plate 3 Gasket 5 Positive electrode 5a Positive electrode lead 6 Negative electrode 6a Negative electrode lead 7 Separator 8a Upper insulating plate 8b Lower insulating plate 9 Electrode group 11 Negative electrode collector 12 First layer 13 Second layer

Claims (5)

A current collector, a first layer formed on a surface of the current collector, and a second layer laminated on the first layer, the first layer including inorganic solid oxide particles, The second layer contains a carbon material, and when the thickness of the first layer is T ₁ and the thickness of the second layer is T ₂ , T ₂ is thicker than T _1. Negative electrode for secondary battery. Wherein the thickness _{T 1} of the first layer, wherein the ratio _T 1 / _{T 2} of the the thickness _{T 2} of the second layer, characterized in that 0.15 to 0.55, according to claim 1 Non according Negative electrode for water electrolyte secondary battery.   The negative electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the inorganic solid oxide particles are lithium titanate having a spinel crystal structure. 4. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein a thickness T ₁ of the first layer is 1 μm or more and 30 μm or less.   The nonaqueous electrolyte secondary battery using the negative electrode for nonaqueous electrolyte secondary batteries of any one of Claim 1 to 4.