JP5659993B2 - Secondary battery - Google Patents

Description

  The present invention relates to a secondary battery obtained by winding or laminating a positive electrode plate and a negative electrode plate.

  Conventionally, there is a secondary battery in which a positive electrode plate and a negative electrode plate are wound or laminated. In such a secondary battery, an insulating member having ion permeability is disposed between the positive electrode plate and the negative electrode plate. Conventionally, many film-like separators have been used as insulating members. For example, Patent Document 1 discloses a porous film containing polyolefin and polyethylene that can be used as a separator for a secondary battery.

  In order to use such a porous film as a separator for a wound type secondary battery, the porous film is required to have a strength that can be wound together with the electrode plate. Polyolefin, which is often used as a separator material, has a higher strength as its molecular weight increases. As a secondary battery separator, polyolefin having a molecular weight of 400,000 or more is usually selected.

  As the separator, many separators that also have a current blocking function by closing holes that allow ions to pass through when the temperature rises to a certain degree or more are used. For example, a porous film having a three-layer structure in which polyethylene is sandwiched between polypropylenes interrupts the current by melting polyethylene. Since the melting point of polyolefin has a correlation with the molecular weight, the current cutoff temperature is within a range of about 130 ° C. ± 5 ° C. in a separator using polyethylene having a molecular weight of 400,000 or more under the above conditions.

JP 2008-106237 A

  The temperature at which volatilization / decomposition of an electrolytic solution generally used in a lithium ion secondary battery starts is approximately 130 ° C. In addition, the temperature inside the secondary battery is not uniform. Therefore, at the current cutoff temperature (about 130 ° C. ± 5 ° C.) of the conventional separator described above, there is a possibility that the volatilization / decomposition of the electrolyte starts depending on the location before the current cutoff due to the melting of the separator occurs.

  Alternatively, if the temperature rises a little further after the current interruption, the volatilization / decomposition of the electrolyte may proceed further. In other words, it was desired to cut off the current at a slightly lower temperature than to cut off the current at around 130 ° C. However, the current interruption temperature must be sufficiently higher than about 60 ° C., which is the normal operating range. Specifically, it is desirable to interrupt the current within a range of about 80 to 115 ° C.

  The present invention has been made to solve the problems of the conventional secondary battery described above. That is, the problem is to provide a secondary battery in which current is interrupted by a temperature rise in the range of about 80 to 115 ° C.

The secondary battery of the present invention made for the purpose of solving this problem is a secondary battery having an electrode body in which a positive electrode plate and a negative electrode plate are overlapped, and at least one of the positive electrode plate and the negative electrode plate is , Metal foil, electrode active material layer formed on the surface of the metal foil, and resin fine particle layer formed on the electrode active material layer. The resin fine particle layer has a weight average molecular weight of GPC. 90% by weight or more of the particles constituting the resin fine particle layer are particles having a particle diameter of 1 to 10 μm. is occupied, the thickness of the resin fine particle layer, is shall der range of 10 to 100 [mu] m.

According to the secondary battery of the present invention, the positive electrode plate or the negative electrode plate has the resin fine particle layer. Furthermore, the polyethylene particles constituting the resin fine particle layer have a weight average molecular weight in the range of 5000 to 400,000 in terms of a relative value based on the molecular weight of polystyrene by the GPC method. The melting temperature of such polyethylene particles is in the range of 80 to 115 ° C. That is, when the temperature of the secondary battery of the present invention is in the range of 80 to 115 ° C., the polyethylene particles constituting the resin fine particle layer are melted and deformed, and the gaps between the particles are closed. In this state, since the ion permeability of the resin fine particle layer is lowered, the current is interrupted. As a result, the secondary battery of the present invention cuts off the current when the temperature rises to a range of about 80 to 115 ° C. Moreover, if it is such, it can be set as the electrode plate which has both insulation performance and ion permeability appropriately.

Further, in the present invention, dendritic fat particle layer is desirably one that is formed on the negative electrode plate.

  According to the secondary battery of the present invention, the current is interrupted by the temperature rise to the range of about 80 to 115 ° C.

It is explanatory drawing which shows the negative electrode plate which concerns on this form. It is a graph which shows the relationship between temperature and impedance. It is a graph which shows the relationship between a weight average molecular weight and resistance rise temperature.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In this embodiment, the present invention is applied to a lithium ion secondary battery.

  The secondary battery of the present embodiment has a positive electrode plate and a negative electrode plate, which are wound in layers and enclosed in a case together with an electrolyte. For example, as shown in FIG. 1 of Japanese Patent Application Laid-Open No. 2007-053055. The positive electrode plate of this embodiment is one in which a positive electrode active material layer is formed on both surfaces of an aluminum foil. The positive electrode active material layer includes a positive electrode mixture of a positive electrode active material capable of occluding and releasing lithium ions. For example, a lithium-containing metal oxide kneaded with a binder and a dispersion solvent is preferable. is there. The electrolyte is preferably a non-aqueous electrolyte containing a lithium salt or an ion conductive polymer. The positive electrode plate and the electrolytic solution may be general ones that have been used conventionally.

  The negative electrode plate 11 of this embodiment has a copper foil 21, a negative electrode active material layer 22, and a resin fine particle layer 23 as schematically shown only on one side in FIG. 1. The negative electrode active material layer 22 contains a carbon material or the like. Further, a resin fine particle layer 23 is formed on the negative electrode active material layer 22. The resin fine particle layer 23 is fixed on the negative electrode active material layer 22, and the copper foil 21, the negative electrode active material layer 22, and the resin fine particle layer 23 are integrally formed into a strip-like negative electrode plate 11.

  Actually, in the negative electrode plate 11 of this embodiment, the negative electrode active material layer 22 and the resin fine particle layer 23 are formed on both surfaces of the copper foil 21. That is, the surface of the negative electrode plate 11 is covered with the resin fine particle layer 23.

  The resin used for the resin fine particle layer 23 of the negative electrode plate 11 of this embodiment is a fine particle of polyethylene (PE) having a molecular weight in the range of 5000 to 400000. The molecular weight referred to in the present embodiment is a weight average molecular weight and is a relative value based on polystyrene obtained by GPC (Gel Permeation Chromatography) method. As such a particulate resin, for example, “Chemical” (trade name) manufactured by Mitsui Chemicals, which is provided in the form of a suspension in which resin particulates are dispersed in water, can be used. The PE fine particles used in this embodiment are preferably those having a nominal value by the manufacturer and having an average particle size in the range of 1 to 10 μm.

  For example, a resin fine particle may be attached to the surface of the negative electrode active material layer 22 by applying this suspension on both sides of the copper foil 21 on which the negative electrode active material layer 22 is formed and drying it. it can. At this time, resin fine particles also adhere to each other to form a layer. As a result, the resin fine particle layer 23 is formed as shown in FIG. The fine particles contained in the resin fine particle layer 23 overlap the negative electrode active material layer 22 as a whole, although many gaps remain between them. Therefore, there is no portion where the negative electrode active material layer 22 is exposed on the surface.

  Therefore, even if this negative electrode plate 11 is overlapped with a general positive electrode plate and brought into contact with each other, the negative electrode active material layer 22 of the negative electrode plate 11 and the positive electrode active material layer of the positive electrode plate do not contact each other. That is, the negative electrode plate 11 and the positive electrode plate are insulated by the resin fine particle layer 23. Therefore, the resin fine particle layer 23 functions as an insulating member.

  Furthermore, the fine particles in the resin fine particle layer 23 are fixed while maintaining the original particle shape (here, substantially spherical), and as shown in FIG. Many remain. Therefore, lithium ions can pass through this gap. Therefore, the resin fine particle layer 23 has ion permeability.

  Since fine particles having a particle diameter in the range of 1 to 10 μm are used, the resin fine particles hardly penetrate into the negative electrode active material layer 22. If the particle size is too small, the gap between the particles becomes small, which may hinder ion permeability, which is not preferable. Here, the particle size of 1 to 10 μm means that the proportion of particles constituting the resin fine particle layer 23 within the range of the particle size of 1 to 10 μm is 90% or more by weight. Not all particles have a particle size within this range.

  The secondary battery of this embodiment is manufactured by stacking a negative electrode plate 11 and a general positive electrode plate, and enclosing them in a case together with an electrolytic solution. When the secondary battery is heated to a temperature equal to or higher than the melting temperature of the resin in the resin fine particle layer 23, the fine particles in the resin fine particle layer 23 are melted and deformed, and the gap between the fine particles is closed. As a result, the ion permeability of the resin fine particle layer 23 is greatly reduced, and the current of the secondary battery is cut off. Therefore, the resin fine particle layer 23 has a function of interrupting current when the temperature is raised. However, since the resin fine particle layer 23 of this embodiment is integrated with the negative electrode plate 11, even if it is melted in this way, it does not shrink in the surface direction unlike a conventional film-like separator.

  Moreover, since the resin fine particle layer 23 of this embodiment is integrated with the negative electrode plate 11, the strength for winding is not required alone. Therefore, a resin having a relatively small molecular weight can be used as the material. The resin fine particle layer 23 of this embodiment is formed of PE fine particles having a molecular weight in the range of 5,000 to 400,000. Since the melting temperature of PE particles having a molecular weight in the range of 5,000 to 400,000 is in the range of 80 to 115 ° C., the current cutoff temperature of the secondary battery using the negative electrode plate 11 having the resin fine particle layer 23 is It is in the range of 80 to 115 ° C.

  That is, the secondary battery of this embodiment can select the current cutoff temperature according to the molecular weight of the resin used for the resin fine particle layer 23. The selectable range is considerably wider than that of conventional film separators. In the negative electrode plate 11 of this embodiment, PE having a molecular weight in the range of 5,000 to 400,000 is employed, so that a current cutoff temperature in an appropriate range can be obtained. In addition, since the resin fine particle layer 23 of the negative electrode plate 11 exhibits the function of a conventional separator, the secondary battery of this embodiment does not have a film-like separator.

  The layer thickness of the resin fine particle layer 23 is suitably in the range of 10 to 100 μm. The resin fine particle layer 23 thinner than the average particle diameter cannot be appropriately formed. If the layer thickness is too thick, the negative electrode plate 11 becomes too thick, which is not preferable.

  The inventor made a simulated secondary battery for experiment using the negative electrode plate 11 on which the resin fine particle layer 23 made of PE having a molecular weight of 250,000 was formed, and examined the relationship between the temperature and the impedance value between both electrodes. As a result, as indicated by a solid line L1 in FIG. 2, it was found that the impedance rapidly increased in a specific temperature range, and there was a temperature range in which the impedance hardly changed before and after that. In this example, the temperature at which the impedance rises rapidly is about 104 to 108 ° C. This temperature corresponds to a temperature at which the ion permeability of the resin fine particle layer 23 decreases. In the graph of this figure, the vertical axis represents the logarithmic axis. Below, the temperature at which this impedance rises rapidly is called the resistance rise temperature.

  Furthermore, the inventor investigated the relationship between the temperature and the impedance value between both electrodes in the same manner for a conventional secondary battery in which a film-like separator is disposed between the positive electrode plate and the negative electrode plate. This separator is a film separator having a three-layer structure containing PE having a molecular weight of 500,000. The result was as shown by the solid line L2 in FIG. That is, the result that the temperature at which the impedance suddenly rises is considerably higher than that of the solid line L1, although there are places where the impedance suddenly rises as in the case of the solid line L1. The resistance rise temperature in this example was about 130 to 132 ° C.

  From this experiment, it was confirmed that the resistance rise temperature of the secondary battery depends on the molecular weight of the PE used as the material of the resin fine particle layer 23 or the separator. Therefore, the present inventor next manufactured a secondary battery using the negative electrode plate 11 having the resin fine particle layer 23 of PE having a different molecular weight, and investigated the relationship between the molecular weight and the resistance increasing temperature. The resistance rise temperature was also measured for the film separator. In this experiment, the temperature at which the internal resistance of the secondary battery became 50 times or more from the initial state was defined as the resistance increase temperature.

  In this experiment, as Examples 1 to 4, the resin fine particle layer 23 using PE particles having a weight average molecular weight of 5,000, 40,000, 250,000, and 400,000 was manufactured. In addition, the measuring method of the weight average molecular weight Mw in this experiment is mentioned later. Each of these resin fine particle layers 23 uses PE fine particles having an average particle diameter of 2.5 μm, and is obtained by adding 0.3% CMC (carboxymethylcellulose) as a thickener to 99.7% PE fine particles. Manufactured. The thickness of each resin fine particle layer 23 was 30 μm.

  Further, as Comparative Examples 1 and 2, two types of secondary batteries using film separators having different molecular weights were prepared. Furthermore, a secondary battery using the negative electrode plate 11 having the resin fine particle layer 23 using PE particles having a molecular weight of 3,000 which is outside the scope of the present invention was manufactured in the same manner as in Example, and this was designated as Comparative Example 3.

  The results of this experiment are shown in Table 1 above and FIG. The relationship between the weight average molecular weight of PE used in the resin fine particle layer and the resistance increase temperature was such that the resistance increase temperature increased as the weight average molecular weight increased, as indicated by the solid line L3 in FIG. In all of Examples 1 to 4, the resistance increase temperature was in the range of 80 to 115 ° C. Therefore, it was confirmed that the present invention is suitable as a form of the present invention.

  On the other hand, in Comparative Examples 1 and 2, the resistance rise temperature was too high. In Comparative Example 3, the resistance rise temperature was too low. Also from this experiment, by using the resin fine particle layer 23 formed of PE particles having a molecular weight of 5,000 to 400,000 within the range of the present embodiment, a secondary battery having a resistance increase temperature of 80 to 115 ° C. is obtained. It was confirmed that it was possible.

  Next, a method for measuring the molecular weight of PE in this embodiment will be described. Although there are various methods for measuring the molecular weight of the resin, the molecular weight referred to in this embodiment is a relative value with the standard sample as standard polystyrene and the molecular weight of polystyrene as a reference. That is, this measurement result was prepared by approximating the relationship between the logarithm of the molecular weight of polystyrene and the elution time by a cubic equation. The measurement conditions are as described below. This measurement is a general measurement method for measuring the molecular weight of PE. Furthermore, in this embodiment, the weight average molecular weight (Mw) calculated by the following formula using the measurement result is simply called the molecular weight.

Apparatus: High temperature GPC apparatus (PL-220 manufactured by Polymer Laboratories)
Detector: Differential refractive index detector RI
Column: Shodex UT-G + HT-806M (two)
Solvent: 1,2,4-trichlorobenzene (TCB, Wako Pure Chemical Industries, Ltd.) (0.1% BHT added)
Flow rate: 1.0 mL / min
Column temperature: 145 ° C
Sample preparation: 5 mL of GPC measurement solvent was added to 5 mg of the sample, and the mixture was heated and stirred at 150 to 160 ° C. for about 20 minutes.
Injection volume: 0.200 mL
Standard sample: Standard polystyrene

Furthermore, in this embodiment, the weight average molecular weight (Mw) defined as the following formula was used.
Weight average molecular weight Mw = Σ (Ni · Mi ² ) / Σ (Ni · Mi)
here,
Mi is the molecular weight Ni at each elution position of the GPC curve obtained through the molecular weight calibration curve, and Ni is the number of molecules.

  As described in detail above, according to the secondary battery of the present embodiment, since the resin fine particle layer 23 is provided on the negative electrode plate 11, the insulation performance and the temperature increase can be achieved without using a film-like separator. Has current interruption capability. In particular, since the resin fine particle layer 23 is made of PE particles having a weight average molecular weight in the range of 5000 to 400,000 relative to the molecular weight standard of polystyrene by the GPC method, the resistance increase temperature is 80 to 115. Within the range of ° C. Therefore, the secondary battery of this embodiment is one in which current is interrupted by a temperature rise up to a range of 80 to 115 ° C.

In addition, this form is only a mere illustration and does not limit this invention at all. Therefore, the present invention can naturally be improved and modified in various ways without departing from the gist thereof.
For example, in the present embodiment, the resin fine particle layer 23 is provided on the negative electrode plate 11 because it is easy to process, or the secondary battery generally uses a negative electrode plate that is slightly larger than the positive electrode plate. Depending on the reason, it may be provided on both surfaces of the positive electrode plate. Or it is good also as providing one surface each in a negative electrode plate and a positive electrode plate. Further, in this embodiment, the PE particles added with CMC are applied, but other materials may not be added. For example, resin particles other than PE, an adhesive, or the like may be added. Also, it should not be used with a film separator. Further, the present invention is not limited to the wound type but can be applied to a stacked type secondary battery.

DESCRIPTION OF SYMBOLS 11 Negative electrode plate 21 Copper foil 22 Negative electrode active material layer 23 Resin fine particle layer

Claims (2)

In a secondary battery having an electrode body formed by stacking a positive electrode plate and a negative electrode plate,
At least one of the positive electrode plate and the negative electrode plate is
Metal foil,
An electrode active material layer formed on the surface of the metal foil;
A resin fine particle layer formed on the electrode active material layer,
The resin fine particle layer is composed of polyethylene particles having a weight average molecular weight within a range of 5000 to 400,000 in terms of a relative value of polystyrene molecular weight based on GPC method ,
90% by weight or more of the particles constituting the resin fine particle layer is occupied by particles having a particle diameter in the range of 1 to 10 μm
Secondary battery layer thickness of the resin particle layer, characterized in der Rukoto the range of 10 to 100 [mu] m. The secondary battery according to claim 1 ,
Secondary battery, wherein the pre-Symbol resin fine particle layer, those which are formed on the negative electrode plate.

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