JP2011103181A - Lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a battery capable of preventing failure from occurring due to thermal decomposition of a positive electrode active material at its failures. <P>SOLUTION: The lithium secondary battery includes an electrode body equipped with a positive electrode 66 and a negative electrode, and a case housing the electrode body and electrolyte. The positive electrode includes a positive electrode layer 64 mainly composed of a positive electrode active material 67 formed on a positive electrode collector 62, and the negative electrode includes a negative electrode layer mainly composed of a negative electrode active material formed on a negative electrode collector. At least either of the electrode layers of the positive electrode layer and the negative electrode layer contains an endothermic material 69 made of a substance showing a phase change from solid to liquid at a given temperature, and the endothermic material is of a double structure having its surface coated with an oxide film made of an oxide of a matter constituting it, and is so structured to flow into the inside of the electrode layer by performing a phase change from solid to liquid when a temperature of the electrode layer reaches a given height. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery including a heat absorbing material that suppresses battery temperature rise when the battery is misused such as internal short circuit or overcharge.

  A lithium secondary battery (for example, a lithium ion battery) that is charged and discharged by moving lithium ions back and forth between the positive electrode and the negative electrode is lightweight and has a high energy density. The importance is increasing as what is preferably used for the power supply of a portable terminal.

When charging a lithium secondary battery, if the battery is overcharged due to malfunction due to the presence of a defective battery or a failure of the charger, the battery may be overcharged, or an internal short circuit may cause It is assumed that a short-circuit current flows in In the case of such overcharge, the battery reaction rapidly proceeds, gas is generated inside the battery case, the internal pressure inside the case rises, and the electrolytic solution decomposes on the surface of the active material or the electrode The temperature inside the battery may rise due to the heat generated in the active material. At this time, if the battery is provided with a safety valve or the like, the gas is discharged to the outside by the operation of the safety valve or the like, thereby preventing problems such as deformation of the case and interrupting the current.
However, even if the gas generated inside is discharged outside and the battery current is cut off, the battery reaction continues and the temperature of the active material (typically the positive electrode active material) itself becomes very high. In this case, the thermal decomposition reaction of the positive electrode active material itself progresses, and the temperature inside the battery suddenly rises, so that there is a possibility that the battery itself may malfunction. In order to cope with such an abnormality, Patent Documents 1 and 2 can be cited as conventional techniques. Patent Document 1 describes a technique for suppressing an increase in temperature in a battery in an emergency by adding an endothermic substance accompanying a phase change in a positive electrode layer. Patent Document 2 discloses that a battery in which a material accompanied with a phase change is placed in an inert material capsule is added to an electrode active material to effectively suppress a temperature increase during normal and abnormal battery operation. The technology which tries to improve the life and safety of is described.

Japanese Patent Laid-Open No. 11-040200 JP 2008-509519 A

However, in the techniques described in the above patent documents, there is a possibility that the endothermic substance or the like that has been added in advance may be altered or dissolved by the battery reaction during normal use of the battery. As a result, there is a possibility that the endothermic substance or the like does not act properly at the time of an abnormality in which the temperature inside the battery rises due to overcharging or the like, and inconvenience due to thermal decomposition of the positive electrode active material cannot be prevented.
Therefore, the present invention has been created to solve the above-described conventional problems, and its purpose is to prevent the occurrence of problems due to thermal decomposition of the positive electrode active material in the event of battery abnormality. It is to provide a battery.

  According to the present invention, there is provided a lithium secondary battery including an electrode body including a positive electrode and a negative electrode, and a case containing the electrode body and an electrolyte. In the lithium secondary battery disclosed herein, the positive electrode includes a positive electrode layer mainly composed of a positive electrode active material formed on a positive electrode current collector, and the negative electrode is formed on the negative electrode current collector. And a negative electrode layer mainly composed of the negative electrode active material. At least one of the positive electrode layer and the negative electrode layer (hereinafter, the positive electrode layer and the negative electrode layer are collectively referred to as an electrode layer) is a substance that exhibits a phase change (phase transition) from solid to liquid at a predetermined temperature. Contains an endothermic material. The endothermic material has a double structure in which the surface is covered with an oxide film made of an oxide of the substance constituting the endothermic material, and the temperature of the electrode layer including the endothermic material reaches the predetermined temperature. In this case, the endothermic material is configured to change phase from solid to liquid and to flow out to the inner region of the electrode layer.

In the present specification, the “lithium secondary battery” refers to a secondary battery that uses lithium ions as electrolyte ions and is charged and discharged by movement of lithium ions between the positive and negative electrodes. A secondary battery generally referred to as a lithium ion battery is a typical example included in the lithium secondary battery in this specification.
In this specification, the “positive electrode active material” means a positive electrode side capable of reversibly occluding and releasing (typically inserting and removing) chemical species (here, lithium ions) which are charge carriers in a secondary battery. The active material.
Furthermore, in this specification, the “negative electrode active material” refers to a negative electrode capable of reversibly occluding and releasing (typically inserting and removing) chemical species (here, lithium ions) that serve as charge carriers in a secondary battery. The active material on the side.

The lithium secondary battery provided by the present invention includes an endothermic material made of a substance that exhibits a phase change from solid to liquid at a predetermined temperature in at least one of the positive electrode layer and the negative electrode layer, The surface of the endothermic material has a double structure (typically a structure composed of a metal material and an oxide film of the metal material) coated with an oxide film composed of an oxide of the material constituting the endothermic material. When the temperature of the electrode layer including the endothermic material reaches the predetermined temperature, the endothermic material changes phase from solid to liquid and flows out to the inner region of the electrode layer.
Thus, the endothermic material contained in the electrode layer has a double structure covered with an oxide film made of an oxide of the endothermic material. Therefore, the charge / discharge potential region during typical use of a lithium secondary battery (typically Specifically, it is possible to prevent the endothermic material from being dissolved and dissolved out as ions at 3.0 V to 4.1 V). When the temperature of the electrode layer including the endothermic material reaches a predetermined temperature (melting point of the endothermic material) in the event of a battery abnormality, the endothermic material absorbs heat from decomposition of the active material and the electrolytic solution and becomes liquid from solid Therefore, a further temperature increase of the electrode layer can be suppressed. Furthermore, the endothermic material contained in the electrode layer undergoes a phase change from solid to liquid, and the liquefied (molten) endothermic material flows out into the inner region of the electrode layer, thereby causing the endothermic material in the molten (liquid) state. The material covers the surface of the active material. As a result, the surface area of the active material is reduced, so that the decomposition reaction of the electrolyte solution on the surface of the active material is stopped or reduced, so that heat generation due to the decomposition reaction is suppressed and lithium ions are released and / or occluded from the surface of the active material. Since the current flow is blocked and the current flow is shut down, heat generation in the active material can be suppressed (by this, the exothermic reaction in the active material converges).
Therefore, according to the present invention, it is possible to provide a lithium secondary battery that is excellent in safety and reliability without causing trouble in the battery due to a rapid thermal decomposition reaction of the active material in the event of battery abnormality.

  In a preferred aspect of the secondary battery disclosed herein, the endothermic material is composed of a material that melts at a temperature lower than a temperature at which the positive electrode active material undergoes a thermal decomposition reaction. By including in the electrode layer an endothermic material having a melting temperature (melting point) lower than the temperature at which the positive electrode active material undergoes a thermal decomposition reaction, even if the temperature of the electrode layer rises in the event of a battery abnormality, the endothermic material increases the temperature. Therefore, the temperature at which the thermal decomposition reaction of the positive electrode active material occurs is not reached, and it is possible to prevent the occurrence of a battery failure due to the thermal decomposition reaction.

  In a preferred aspect of the secondary battery disclosed herein, the endothermic material is composed of a metal material, particularly tin or a solder alloy containing tin as a main component. The melting point of tin or a solder alloy containing tin as a main component is relatively low, and is about the same as or lower than the temperature at which the active material contained in the electrode layer undergoes a thermal decomposition reaction. Therefore, even when the battery is abnormal such as overcharge, the temperature rise of the electrode layer can be suppressed by the phase change of the endothermic material before the thermal decomposition reaction of the active material occurs, or the thermal decomposition reaction of the active material has occurred. However, the temperature increase of the electrode layer can be suppressed immediately by the phase change of the endothermic material. Further, since the liquefied (molten) endothermic material covers the surface of the active material, the battery reaction is shut down, so that it is possible to prevent the battery from being defective.

  In a preferred aspect of the secondary battery disclosed herein, the endothermic material is included in at least the positive electrode layer. The positive electrode active material has a lower temperature at which the thermal decomposition reaction of the active material itself occurs than the negative electrode active material. Therefore, the thermal decomposition reaction of the positive electrode active material in the positive electrode layer that can occur in a lower temperature region than the negative electrode layer can be more effectively prevented by including the endothermic material having the double structure in the positive electrode layer.

  Further, according to the present invention, there is provided a vehicle including any of the lithium secondary batteries disclosed herein (typically, lithium ion batteries). The lithium secondary battery provided by the present invention may exhibit quality suitable as a lithium secondary battery mounted on a vehicle (for example, further improvement in safety and reliability of the lithium secondary battery). Therefore, the lithium secondary battery can be suitably used as a power source for a motor (electric motor) mounted on a vehicle such as an automobile equipped with an electric motor such as a hybrid vehicle, an electric vehicle, and a fuel cell vehicle.

It is a perspective view which shows typically the external shape of the lithium secondary battery which concerns on one Embodiment. It is a longitudinal cross-sectional view which follows the II-II line | wire in FIG. It is a schematic diagram which shows the positive electrode layer apply | coated to the positive electrode electrical power collector which concerns on one Embodiment. It is a schematic diagram which shows the negative electrode layer apply | coated to the negative electrode collector which concerns on one Embodiment. It is a graph which shows the relationship between SOC and battery temperature. It is a side view which shows typically the vehicle (automobile) provided with the lithium secondary battery which concerns on this invention.

  Hereinafter, preferred embodiments of the present invention will be described. It should be noted that matters other than matters specifically mentioned in the present specification and necessary for carrying out the present invention can be grasped as design matters of those skilled in the art based on the prior art in this field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field.

  The positive electrode provided in the lithium secondary battery disclosed herein can have the same configuration as that of the prior art, except that it includes a heat-absorbing material having characteristics that characterize the present invention. As a positive electrode current collector constituting such a positive electrode, a conductive material made of a metal having good conductivity is used as in the current collector used in the positive electrode of a conventional lithium secondary battery (typically, a lithium ion battery). A member is preferably used. For example, aluminum or an alloy containing aluminum as a main component can be used. The shape of the positive electrode current collector can vary depending on the shape of the lithium secondary battery, and is not particularly limited, and may be various forms such as a rod shape, a plate shape, a sheet shape, a foil shape, and a mesh shape. Typically, a sheet-like positive electrode current collector made of aluminum is used.

  Next, the material constituting the positive electrode layer formed on the surface of the positive electrode current collector will be described. On the surface of the positive electrode current collector, a composition in which a positive electrode active material, a conductive material, a binder, an endothermic material, etc. are mixed in a suitable solvent (typically a composition formed in a paste form, hereinafter It has a positive electrode layer formed using a positive electrode layer forming paste).

As the positive electrode active material included in the positive electrode layer formed on the positive electrode of the lithium secondary battery disclosed herein, a granular active material capable of inserting and extracting lithium is used. A typical positive electrode active material includes a composite oxide containing lithium and at least one transition metal element. For example, cobalt lithium composite oxide (LiCoO ₂ ), nickel lithium composite oxide (LiNiO ₂ ), manganese lithium composite oxide (LiMn ₂ O ₄ ), or nickel-cobalt-based LiNi _x Co _1-x O ₂ ( 0 <x <1), cobalt / manganese-based LiCo _x Mn _1-x O ₂ (0 <x <1), nickel / manganese-based LiNi _x Mn _1-x O ₂ (0 <x <1) and LiNi as represented by _{x Mn 2-x O 4 (} 0 <x <2), a so-called binary lithium-containing composite oxide containing two kinds of transition metal elements, or nickel cobalt containing three transition metal elements A ternary lithium-containing composite oxide such as manganese may be used.
The positive electrode active material has a general formula of LiMAO ₄ (where M is at least one metal element selected from the group consisting of Fe, Co, Ni and Mn, and A is P, Si, S and A polyanionic compound represented by the following formula is also preferably used: an element selected from the group consisting of V.

  The compound which comprises such a positive electrode active material can be prepared and provided by a conventionally well-known method, for example. For example, the oxide can be prepared by mixing several raw material compounds appropriately selected according to the atomic composition at a predetermined molar ratio and firing the mixture at a predetermined temperature by an appropriate means. Further, the fired product is pulverized, granulated and classified by an appropriate means to obtain a granular positive electrode active material powder substantially composed of secondary particles having a desired average particle size and / or particle size distribution. be able to. In addition, the preparation method itself of a positive electrode active material (lithium containing complex oxide powder etc.) does not characterize this invention at all.

  In addition, the conductive material included in the positive electrode layer formed in the positive electrode disclosed herein is not limited to a specific conductive material as long as it is conventionally used in this type of secondary battery. For example, carbon materials such as carbon powder and carbon fiber can be used. As the carbon powder, various carbon blacks (for example, acetylene black, furnace black, ketjen black), graphite powder, and the like can be used. You may use 1 type or 2 types or more together among these.

Further, as the endothermic material contained in the positive electrode layer formed in the positive electrode disclosed herein, a substance that changes in phase from a solid to a liquid at a predetermined temperature, for example, a relatively low melting point (for example, a melting point of 250 ° C. or lower, preferably 200 ° C. to 250 ° C. or 200 ° C. or lower) and various alloy materials containing the metal as a main component. Examples of the metal material having a melting point of 250 ° C. or lower or 200 ° C. or lower include a solder material, preferably tin (Sn) or a solder alloy mainly composed of tin. As the solder alloy, lead-free solder (lead-free solder) is preferable in consideration of the influence on the natural environment and the human body. The solder material is preferably a material having a melting point lower than the temperature at which the thermal decomposition reaction of the positive electrode active material proceeds (thermal decomposition temperature, typically a temperature range of about 200 ° C. to 300 ° C.). As solder materials, Sn (melting point: 232 ° C.), Sn—Cu type (melting point: 227 ° C.), Sn—Ag type (melting point: 221 ° C.), Sn—Ag—Cu type (melting point: 217 ° C. to 219 ° C.), Sn—Ag -Cu-Bi system (melting point 211 ° C to 221 ° C), Sn-Ag-Bi-In system (melting point 170 ° C to 206 ° C), Sn-Zn system (melting point 199 ° C), Sn-Bi system (melting point 139 ° C) Sn-Cu-Ni system (melting point 227 ° C), Sn-Pb system (melting point 183 ° C), and the like. Note that bismuth (Bi), indium (In), or a solder alloy containing these as a main component may be used.
The amount of the endothermic material (typically a solder material) is appropriately determined, but it is preferably used in an amount that does not hinder the conductivity of the positive electrode layer, that is, about 1 to 30 parts by mass with respect to 100 parts by mass of the positive electrode active material. More preferably, it is about 5-15 mass parts.
The average particle diameter based on observation of an endothermic material (solder material) by an electron microscope (such as SEM or TEM) (or based on a light scattering method) is preferably within a range of 0.1 μm to 100 μm, and more preferably. Is in the range of 1 μm to 10 μm.

Generally, when the endothermic material is not entirely covered with an oxide film, the endothermic material is configured in a normal charge / discharge potential range (typically 3.0 V to 4.1 V) of the lithium secondary battery. The substance to be dissolved becomes ions and dissolves in the electrolyte. For this reason, the endothermic material dissolved in the electrolyte as ions cannot absorb the heat in the case (typically the heat of the electrode layer) when the battery is abnormal such as overcharge. There is a risk that the decomposition reaction proceeds and the temperature of the electrode layer rises rapidly, causing a problem in the battery. However, in the present invention, the endothermic material (typically a solder material) has a double structure in which the surface is coated with an oxide film made of an oxide of a substance constituting the endothermic material (the entire endothermic material is A state in which the composition of the endothermic material (that is, the inside of the oxide film) and the outside (for example, air, electrolyte, etc.) are not in direct contact with each other. For this reason, the endothermic material does not dissolve in the electrolyte solution in the normal charge / discharge potential region of the lithium secondary battery, and the battery layer can absorb heat when the battery is abnormal, causing problems in the battery. It can be prevented in advance.
The endothermic material having the above average particle diameter can be produced by various conventionally known methods. For example, it can be produced by various liquid phase methods such as spray pyrolysis and various gas phase methods such as chemical vapor deposition.
As a method for forming an oxide film on the surface of the endothermic material, for example, a method in which a powder of the endothermic material is heat-treated in an oxygen atmosphere at a temperature lower than the melting point of the endothermic material. The oxide film can also be formed by a technique similar to a conventionally known method such as a sol-gel method or a coating method.

  Furthermore, as a binder (binder) contained in the positive electrode layer formed in the positive electrode disclosed herein, for example, when an aqueous liquid composition is used as the composition for forming the positive electrode layer, Polymer materials that dissolve or disperse can be preferably employed. Cellulose polymers such as carboxymethylcellulose (CMC), methylcellulose (MC), cellulose acetate phthalate (CAP), hydroxypropylmethylcellulose (HPMC), etc .; polyvinyl alcohol (PVA) And the like are exemplified. Examples of polymer materials that are dispersed in water (water dispersible) include fluorine resins such as polytetrafluoroethylene (PTFE); vinyl acetate copolymers; rubbers such as styrene butadiene rubber (SBR); The Alternatively, when a solvent-based liquid composition (typically a composition prepared in the form of a paste or slurry, hereinafter referred to as a positive electrode layer forming paste) is used as a composition for forming the positive electrode layer, a polyfluoride is used. Polymer materials such as vinylidene chloride (PVDF) and polyvinylidene chloride (PVDC) can be used. In addition, the polymer material illustrated above may be used as a thickener and other additives in the above composition in addition to being used as a binder.

  Here, the “aqueous liquid composition” is a concept indicating a composition using water or a mixed solvent mainly composed of water (aqueous solvent) as a dispersion medium of the active material. As the solvent other than water constituting the mixed solvent, one or more organic solvents (lower alcohol, lower ketone, etc.) that can be uniformly mixed with water can be appropriately selected and used. The “solvent-based liquid composition” is a concept indicating a composition in which a dispersion medium of an active material is mainly an organic solvent (non-aqueous solvent). As the organic solvent, for example, N-methylpyrrolidone (NMP) can be used.

The positive electrode disclosed here can be suitably manufactured, for example, generally by the following procedure. A positive electrode layer forming paste is prepared by dispersing the above-described positive electrode active material, conductive material, endothermic material, binder material soluble in an organic solvent, and the like in an organic solvent. The prepared paste is applied to a positive electrode current collector, dried, and then compressed (pressed) to produce a positive electrode including the positive electrode current collector and a positive electrode layer formed on the positive electrode current collector. be able to.
Here, manganese lithium composite oxide (positive electrode layer using LiMn ₂ O ₄ as a positive electrode active material) is relatively excellent in thermal stability, and its thermal decomposition temperature is typically about 300 ° C. to 330 ° C. . The thermal decomposition temperature of the cobalt lithium composite oxide is typically about 240 ° C. to 270 ° C., and the thermal decomposition temperature of the nickel lithium composite oxide is typically about 210 ° C. to 230 ° C.

Next, each component of the negative electrode of the lithium secondary battery disclosed here will be described. The negative electrode disclosed here is a negative electrode for a lithium secondary battery including a negative electrode current collector and a negative electrode layer formed on the current collector. As the negative electrode current collector constituting such a negative electrode, for example, copper, nickel, or an alloy containing them as a main component can be used.
The negative electrode active material contained in the negative electrode layer formed in the negative electrode disclosed herein may be any material that can occlude and release lithium. For example, carbon materials such as graphite, lithium / titanium Examples thereof include oxide materials such as oxide (Li ₄ Ti ₅ O ₁₂ ), alloy materials composed of alloys such as tin, aluminum (Al), zinc (Zn), and silicon (Si). A typical example is the amount of powdery carbon material made of graphite or the like. In particular, the graphite particles can be a negative electrode active material more suitable for rapid charge / discharge (for example, high-power discharge) because the particle size is small and the surface area per unit volume is large.

The negative electrode layer formed in the negative electrode disclosed herein may contain one or more materials that can be blended in the positive electrode layer, if necessary, in addition to the negative electrode active material. As such materials, various materials that can function as binders and heat absorbing materials as listed as constituent materials of the positive electrode layer can be used in the same manner. The amount of the endothermic material (typically a solder material) is preferably about 1 to 40 parts by mass with respect to 100 parts by mass of the negative electrode active material. More preferably, it is about 5-20 mass parts.
The negative electrode disclosed here can be manufactured by the same method as that for the positive electrode. A paste-like (or slurry-like) composition (hereinafter referred to as a negative electrode layer forming paste) is prepared by dispersing a negative electrode active material, an endothermic material, a binder and the like in an appropriate solvent. The prepared paste for forming a negative electrode layer is applied to a negative electrode current collector, dried, and then compressed (pressed) to provide a negative electrode current collector and a negative electrode layer formed on the negative electrode current collector. A negative electrode can be produced.

  Moreover, as a separator used with a positive electrode and a negative electrode, the separator similar to the past can be used. For example, a porous sheet (porous film) made of a polyolefin resin can be used. Alternatively, a polymer solid electrolyte can be used as a separator.

As the electrolyte, the same electrolyte as a non-aqueous electrolyte (typically, an electrolytic solution) conventionally used for a lithium secondary battery can be used without particular limitation. Such a nonaqueous electrolyte typically has a composition in which a supporting salt is contained in a suitable nonaqueous solvent. Examples of the non-aqueous solvent include one or two selected from the group consisting of ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), propylene carbonate (PC), and the like. More than seeds can be used. Further, as the supporting salt, for _example, can be used _{LiPF 6, LiBF 4, LiAsF 6} , LiCF 3 SO 3 , etc. One or two or more lithium compounds selected from the group consisting of a (lithium salt).
Moreover, as long as the positive electrode and negative electrode for lithium secondary batteries disclosed here are employed, the shape (outer shape and size) of the lithium secondary battery to be constructed is not particularly limited. The outer package may be a thin sheet type constituted by a laminate film or the like, and the battery outer case may be a cylindrical or cuboid battery, or may be a small button shape.

Hereinafter, one embodiment of a lithium secondary battery including a positive electrode and a negative electrode containing an endothermic material disclosed herein will be described with reference to the drawings. However, the present invention is not intended to be limited to such an embodiment. .
In addition, in the following drawings, the same code | symbol is attached | subjected to the member and site | part which show | plays the same effect | action, and the overlapping description may be abbreviate | omitted. Moreover, the dimensional relationship (length, width, thickness, etc.) in each drawing does not necessarily reflect the actual dimensional relationship.

FIG. 1 is a perspective view schematically showing a lithium secondary battery according to an embodiment. FIG. 2 is a longitudinal sectional view taken along line II-II in FIG.
As shown in FIGS. 1 and 2, the lithium secondary battery 10 according to the present embodiment includes the above-described lithium secondary battery constituent materials (active material for each positive and negative electrode, current collector for each positive and negative electrode, separator, etc.). An electrode body 50 is provided, and a flat rectangular parallelepiped (that is, rectangular) battery case 15 that accommodates the electrode body 50 and a suitable nonaqueous electrolyte (electrolyte).

The case 15 is attached to (for example, welded) the box-shaped case main body 30 in which one of the narrow surfaces of the flat rectangular parallelepiped shape is the opening 20 and the opening 20 is closed. And a lid 25. As a material constituting the case 15, the same material as that used in a general lithium secondary battery can be used as appropriate, and there is no particular limitation. For example, a container made of metal (for example, aluminum or steel), a container made of synthetic resin (for example, polyolefin-based resin), or the like can be preferably used. The case 15 according to the present embodiment is made of, for example, aluminum.
The lid body 25 is formed in a rectangular shape that matches the shape of the opening 20 of the case body 30. Further, the lid body 25 is provided with a positive electrode terminal 60 and a negative electrode terminal 70 for external connection, respectively, and a part of these terminals 60, 70 protrudes from the lid body 25 toward the outside of the case 15. It is formed to do. Similarly to the case of the conventional lithium secondary battery, the lid 25 is provided with a safety valve (not shown) for discharging the gas generated inside the case 15 to the outside of the case 15 when the battery is abnormal. It has been. The safety valve can be used without any limitation as long as it has a mechanism that opens and discharges gas to the outside of the case 15 when the pressure inside the case 15 exceeds a predetermined level.

  As shown in FIG. 2, the lithium secondary battery 10 includes a wound electrode body 50 (hereinafter may be abbreviated as “electrode body 50”) in the same manner as a normal lithium secondary battery. The electrode body 50 is accommodated in the case main body 30 in a posture in which the winding axis is laid down (that is, in the direction in which the opening 20 is positioned in the lateral direction with respect to the winding axis). The electrode body 50 includes a positive electrode sheet (positive electrode) 66 in which a positive electrode layer (electrode layer) 64 is formed on the surface of a long sheet-like positive electrode current collector 62 and a surface of a long sheet-like negative electrode current collector 72. A negative electrode sheet (negative electrode) 76 on which a negative electrode layer (electrode layer) 74 is formed is overlapped with two long sheet-like separators 80 and wound, and the obtained electrode body 50 is crushed from the side surface direction and abducted. By doing so, it is formed into a flat shape.

  Further, in the wound positive electrode sheet 66, the positive electrode current collector 62 is exposed without forming the positive electrode layer 64 at one end portion along the longitudinal direction, while the negative electrode sheet 76 is wound. The negative electrode current collector 72 is exposed at one end along the longitudinal direction without the negative electrode layer 74 being formed. A positive electrode terminal 60 is joined to the exposed end of the positive electrode current collector 62, and is electrically connected to the positive electrode sheet 66 of the wound electrode body 50 formed in the flat shape. Similarly, the negative electrode terminal 70 is joined to the exposed end portion of the negative electrode current collector 72 and is electrically connected to the negative electrode sheet 76. The positive and negative electrode terminals 60 and 70 and the positive and negative electrode current collectors 62 and 72 can be joined by, for example, ultrasonic welding, resistance welding, or the like.

  The material and the member itself constituting the wound electrode body 50 having the above configuration are disclosed herein as a positive electrode (here, positive electrode sheet 66) in which the positive electrode layer disclosed here as a positive electrode is formed on the positive electrode current collector and a negative electrode. The negative electrode layer may be the same as the electrode body of the conventional lithium secondary battery except that a negative electrode (here, the negative electrode sheet 76) in which the negative electrode layer is formed on the negative electrode current collector is adopted. FIG. 3 is a schematic diagram showing the positive electrode layer 64 applied to the positive electrode current collector 62 according to one embodiment. FIG. 4 is a schematic diagram showing the negative electrode layer 74 applied to the negative electrode current collector 72 according to one embodiment.

The positive electrode sheet 66 is produced by forming a positive electrode layer 64 on a long positive electrode current collector (for example, a long aluminum foil) 62. That is, a positive electrode active material (for example, LiCoO ₂ ) 67, a conductive material (for example, graphite) 68, an endothermic material (for example, tin) 69, and a binder (for example, PVDF) that is soluble in an organic solvent is used as an organic solvent (for example, NMP). A positive electrode layer forming paste dispersed in is prepared. The prepared paste is applied to the positive electrode current collector 62, dried, and then compressed (pressed) to form the positive electrode layer 64.
Here, as a method of applying the paste to the positive electrode current collector 62, a technique similar to a conventionally known method can be appropriately employed. For example, the paste can be suitably applied to the positive electrode current collector 62 by using an appropriate application device such as a slit coater, a die coater, a gravure coater, or a comma coater. Moreover, as a compression method, conventionally known compression methods such as a roll press method and a flat plate press method can be employed. In adjusting the thickness, the thickness may be measured with a film thickness measuring instrument, and the press pressure may be adjusted to compress a plurality of times until a desired thickness is obtained.

  The negative electrode sheet 76 is produced by forming a negative electrode layer 74 on a long negative electrode current collector (for example, a long copper foil) 72. That is, for forming a negative electrode layer formed by dispersing a negative electrode active material (for example, graphite) 77, an endothermic material (for example, tin) 79 and a binder (for example, PVDF) that is soluble in an organic solvent in an organic solvent (for example, NMP) Prepare paste. The prepared paste is applied to the negative electrode current collector 72, dried, and then compressed (pressed) to form the negative electrode layer 74. Since the formation method itself of the negative electrode layer 74 is the same as that of the positive electrode side, detailed description is omitted.

The positive electrode sheet 66 and the negative electrode sheet 76 produced above are stacked and wound together with two separators (for example, porous polyolefin resin) 80, and the wound electrode body 50 obtained is rolled into the case body 30 with its winding shaft lying sideways. And a non-aqueous electrolyte such as a mixed solvent of EC and DMC (for example, a mass ratio of 1: 1) containing a suitable supporting salt (for example, a lithium salt such as LiPF ₆ ) in an appropriate amount (for example, a concentration of 1 M). After injecting (electrolytic solution), the lithium secondary battery 10 of the present embodiment can be constructed by mounting the lid body 25 in the opening 20 and sealing (for example, laser welding).
The endothermic material is included in at least one of the positive electrode layer and the negative electrode layer, preferably included in the positive electrode layer, and more preferably included in each of the positive electrode layer and the negative electrode layer. . In the present embodiment, each of the positive electrode layer 64 and the negative electrode layer 74 includes endothermic materials (tin) 69 and 79.

Next, functions (actions and effects) of the lithium secondary battery 10 according to the present embodiment at the time of battery abnormality such as overcharge will be described in detail.
When the battery is abnormal such as overcharge of the lithium secondary battery (lithium ion battery) 10, the positive electrode active material 67 causes self-heating, or decomposition of the electrolyte progresses on the surfaces of the positive electrode active material 67 and the negative electrode active material 77. The temperature of the positive electrode layer 64 and the negative electrode layer 74 may increase due to heat generation. As shown in the present embodiment, the endothermic materials contained in the positive electrode layer 64 and the negative electrode layer 74, respectively. 69 and 79 absorb the heat of the positive electrode layer 64 and the negative electrode layer 74, and when the melting point of the endothermic materials 69 and 79 is reached, the phase starts from solid to liquid. While the endothermic materials 69 and 79 undergo a phase change, the endothermic materials 69 and 79 absorb the heat of the positive electrode layer 64 and the negative electrode layer 74, thereby suppressing an increase in temperature of the positive electrode layer 64 and the negative electrode layer 74. Can do. Further, the liquefied (melted) endothermic materials 69 and 79 flow into the inner region of the positive electrode layer 64 and the inner region of the negative electrode layer 74 to coat (coat) the surfaces of the positive electrode active material 67 and the negative electrode active material 77, respectively. Since the active materials 67 and 77 whose surfaces are covered (coated) with the endothermic materials 69 and 79 have zero or small surface area in contact with the electrolytic solution, the decomposition reaction of the electrolytic solution on the surfaces of the active materials 67 and 77 is performed. Is stopped or suppressed, and the calorific value becomes zero or decreases. Further, in the positive electrode active material 67 whose surface is coated with the endothermic material 69, lithium ions are not released from the surface, so that the flow of current can be shut down and the temperature rise of the battery 10 can be suppressed. Thereby, since the exothermic reaction of the positive electrode layer 64 and the negative electrode layer 74 converges, the positive electrode active material 67 does not reach the temperature at which the thermal decomposition reaction accompanied by oxygen release proceeds, but is caused by the rapid increase of the temperature accompanying the thermal decomposition reaction. Generation | occurrence | production of the malfunction of the lithium secondary battery 10 can be prevented beforehand.

In another embodiment, the melting point of the endothermic material (for example, Sn) is higher than the temperature at which the thermal decomposition reaction of the positive electrode active material (for example, LiNiO ₂ ) proceeds (for example, about several degrees Celsius to several tens of degrees Celsius). However, by including an endothermic material in at least the positive electrode layer, when the temperature of the positive electrode layer reaches the melting point of the endothermic material, the endothermic material absorbs the heat of the positive electrode layer and changes from a solid to a liquid, Further temperature increase of the positive electrode layer can be suppressed. In addition, the liquefied (molten) endothermic material covers the surface of the positive electrode active material, thereby stopping or reducing the decomposition reaction of the electrolyte solution on the surface of the positive electrode active material and suppressing the thermal decomposition reaction of the positive electrode active material. Since the release of oxygen accompanying the decomposition of the positive electrode active material can be prevented, the reaction with the evaporated organic solvent in the battery case can be suppressed. As a result, the exothermic reaction of the positive electrode layer converges, and it is possible to prevent the occurrence of battery malfunctions associated with the thermal decomposition reaction.

  In order to confirm that by constructing the lithium secondary battery, it is possible to construct a lithium secondary battery excellent in safety and reliability in the event of battery abnormality, the following experiment was conducted as an example. It should be noted that the present invention is not intended to be limited to those shown in the specific examples.

[Construction of Lithium Secondary Battery According to Example]
In this example, a lithium secondary battery (18650 type cell) was constructed as follows. First, the positive electrode of the lithium secondary battery according to the example was produced. That is, tin powder (average particle size of 10 μm or less) as an endothermic material is exposed to 200 ° C. below the melting point of tin (232 ° C.) for 3 hours in an oxygen atmosphere to form an oxide film made of tin oxide on the surface of tin. did. Then, it is weighed so that the mass ratio of these materials is 80: 5: 5: 10 including lithium cobaltate as a positive electrode active material, carbon black, PVDF, and tin (surface tin oxide) coated with the above oxide film. Then, these materials were dispersed in NMP to prepare a positive electrode layer forming paste (paste-like composition). The paste was applied to both sides of a positive electrode current collector (thickness: 15 μm) made of aluminum foil so that the applied amount of the paste was 12 mg / cm ² and dried. After drying, the applied product was pressed to form a positive electrode layer having a positive electrode layer thickness of 80 μm and a positive electrode layer density of 2.5 g / cm ³ . Thus, a sheet-like positive electrode (positive electrode sheet) was produced.
Next, the negative electrode of the lithium secondary battery according to the example was produced. That is, the graphite powder as the negative electrode active material and PVDF as the binder are weighed so that the mass ratio of these materials becomes 93: 7, and these materials are dispersed in NMP to prepare the negative electrode layer forming paste. did. The paste was applied to both sides of a negative electrode current collector (thickness 15 μm) made of copper foil so that the coating amount of the paste was 8 mg / cm ² and dried. After drying, the coated material was pressed to form a negative electrode layer having a negative electrode layer thickness of 40 μm and a negative electrode layer density of 1.4 g / cm ³ . In this way, a sheet-like negative electrode (negative electrode sheet) was produced.

The prepared positive electrode sheet and negative electrode sheet are laminated together with two long separators (here, a porous polypropylene sheet is used), and the laminated sheet is wound in the longitudinal direction to form a wound electrode body. Produced. The electrode body and the electrolyte were housed in a container equipped with a safety valve to construct a cylindrical lithium secondary battery having a diameter of 18 mm and a height of 65 mm (18650 type). As the electrolyte, a nonaqueous electrolytic solution having a composition in which 1 mol / L LiPF ₆ was dissolved in a 3: 7 (volume ratio) mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) was used.

[Construction of lithium secondary battery according to comparative example]
In this comparative example, a lithium secondary battery (18650 type cell) was constructed as follows. First, the positive electrode of the lithium secondary battery which concerns on a comparative example was produced. That is, without using tin (surface tin oxide) coated on the oxide film, the mass ratio of these materials including lithium cobaltate, carbon black, and PVDF as the positive electrode active material is 80: 15: 5. Then, these materials were dispersed in NMP to prepare a positive electrode layer forming paste (paste-like composition). And the positive electrode which concerns on a comparative example was produced in the procedure similar to the said Example.
In this comparative example, a lithium secondary battery according to this comparative example was constructed in the same procedure as the example using the negative electrode similar to the example except that the positive electrode prepared above was used.

[Overcharge test]
Conditioning treatment suitable for the lithium secondary batteries according to the examples and comparative examples constructed as described above (the operation of charging at constant current and constant voltage up to 4.1 V at a charging rate of 0.1 C, and 3 at a discharging rate of 0.1 C) After the initial charge / discharge process of repeating the operation of discharging constant current and constant voltage to 0.0 V three times, the state of charge was adjusted to 80% SOC. Then, the lithium secondary batteries of the example and the comparative example were charged at a constant current of up to 500% SOC at a charging rate of 5C under a temperature condition of room temperature (25 ° C.) (that is, the lithium secondary battery after charging was completed). This is a test in which the charging current is forced to flow to the next battery.) It was confirmed whether or not a malfunction (thermal runaway) occurred in each battery. The results are shown in FIG. In FIG. 5, the horizontal axis represents SOC (%), and the vertical axis represents the battery surface temperature (° C.).

  As shown in FIG. 5, in both the example and the comparative example, the safety valve provided in the case was opened near SOC 185% (portion surrounded by the dotted line in FIG. 5). In the battery according to the comparative example, it was confirmed that the temperature was further increased after the safety valve was opened and the malfunction occurred beyond the temperature at which the thermal decomposition reaction of the positive electrode active material proceeds (the temperature indicated by the dotted line in FIG. 5). It was. On the other hand, although the temperature of the battery according to the example has risen even after the safety valve is opened, it does not reach the temperature at which the thermal decomposition reaction of the positive electrode active material proceeds, and the occurrence of a malfunction (for example, smoke) It was not confirmed. From the above, it is shown that the lithium secondary battery according to the example is a lithium secondary battery that is excellent in safety and reliability without causing malfunction of the battery even when the battery is abnormal due to overcharge. It was done.

  As mentioned above, although this invention was demonstrated by suitable embodiment, such description is not a limitation matter and of course various modifications are possible. For example, the present invention is not limited to the prismatic battery described above, and can be applied to lithium secondary batteries having various shapes (for example, a cylindrical shape). The configuration of the electrode body is not limited to the wound type as described above, and may be, for example, a laminated type electrode body (laminated electrode body) formed by alternately laminating positive and negative electrode sheets together with a separator sheet.

  Since the lithium secondary battery (for example, lithium ion battery) according to the present invention is capable of outputting a large current and is excellent in safety and reliability as described above, it is particularly used for a motor (electric motor) mounted on a vehicle such as an automobile. It can be suitably used as a power source. Therefore, as schematically shown in FIG. 6, the present invention provides a vehicle (typically, a battery (typically, an assembled battery formed by connecting a plurality of such batteries 10 in series) as a power source. Automobiles, in particular automobiles equipped with electric motors such as hybrid cars, electric cars, fuel cars, etc.) 100.

DESCRIPTION OF SYMBOLS 10 Lithium secondary battery 15 Battery case 20 Opening part 25 Cover body 30 Case main body 50 Winding electrode body 60 Positive electrode terminal 62 Positive electrode collector 64 Positive electrode layer (electrode layer)
66 Positive electrode sheet (positive electrode)
67 Positive electrode active material 68 Conductive material 69 Endothermic material 70 Negative electrode terminal 72 Negative electrode current collector 74 Negative electrode layer (electrode layer)
76 Negative electrode sheet (negative electrode)
77 Negative electrode active material 79 Endothermic material 80 Separator 100 Vehicle

Claims (5)

An electrode body comprising a positive electrode and a negative electrode, a case housing the electrode body and the electrolyte,
A lithium secondary battery comprising:
The positive electrode includes a positive electrode layer mainly composed of a positive electrode active material formed on a positive electrode current collector,
The negative electrode includes a negative electrode layer mainly composed of a negative electrode active material formed on a negative electrode current collector,
At least one of the positive electrode layer and the negative electrode layer includes an endothermic material made of a substance that exhibits a phase change from solid to liquid at a predetermined temperature.
The endothermic material has a double structure in which the surface is covered with an oxide film made of an oxide of a substance constituting the endothermic material, and when the temperature of the electrode layer reaches the predetermined temperature, the endothermic material The lithium secondary battery is configured such that the phase changes from solid to liquid and flows into the inner region of the electrode layer.   The lithium secondary battery according to claim 1, wherein the endothermic material is made of a material that melts at a temperature lower than a temperature at which the positive electrode active material undergoes a thermal decomposition reaction.   3. The lithium secondary battery according to claim 1, wherein the endothermic material is made of tin or a solder alloy containing tin as a main component. 4.   The lithium secondary battery according to claim 1, wherein the endothermic material is included in at least the positive electrode layer.   A vehicle comprising the lithium secondary battery according to claim 1.

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