JP2016072098A - Electrode for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrode for a lithium ion secondary battery, which can increase the safety in the event of overcharge without sacrificing various battery characteristics such as current load characteristic and charge and discharge cycle characteristics.SOLUTION: An electrode 10 for a lithium ion secondary battery comprises: a current collector 1; a conductive heat shrinkable layer 2 on the current collector; and a conductive active material layer 3. The heat shrinkable layer 2 includes: a heat shrinkable resin consisting of a polyolefin-based resin such as polypropylene or polyethylene; and at least one material selected from conductive materials consisting of a metal, a metal compound, amorphous carbon, crystal carbon and a conductive polymer.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an electrode, particularly a positive electrode, of a lithium ion secondary battery having a high energy density.

  Lithium ion secondary batteries are superior in light weight and small footprint due to their high energy density, and have the advantage of less memory effect than nickel-cadmium batteries and nickel-hydrogen batteries. Widely used in portable devices such as phones and notebook computers. Moreover, in recent years, it has been increasingly used as a power source to replace fossil fuels such as oil that have been used in automobiles because of its environmental impact. In addition, recently, there are high expectations for stationary storage batteries that are responsible for a part of power supply to homes.

  Commonly used components of a lithium ion secondary battery include an electrode, an electrolytic solution, a separator, a current collector, and an exterior body. Further, the electrode is generally a positive electrode active material or a negative electrode active material, a conductive assistant. It is composed of an agent and a binder. Hereinafter, a mixture of these constituent materials at a predetermined mixing ratio is collectively referred to as a positive electrode material and a negative electrode material, and a positive electrode material and a negative electrode material are collectively referred to as an electrode material. The active material is a material that can insert and desorb lithium ions in the positive electrode and the negative electrode of the lithium ion secondary battery, and plays a role of flowing current by accompanying the exchange of electrons at the time of insertion and desorption. The conductive auxiliary agent is disposed inside the electrode in order to facilitate the electron transfer between the active material and the active material and between the active material and the current collector. The binder is mixed inside the electrode in order to enhance the adhesion between the active material, the conductive additive and the current collector.

  As described above, when a lithium ion secondary battery is used for the purpose of supplying power to an automobile or a home, a larger battery capacity is required as compared with a conventional consumer application such as a mobile phone. Thus, storing a large amount of energy in the battery means that the degree of danger when an abnormality occurs in the battery dramatically increases. In particular, when the battery is charged beyond the original charging potential, in a so-called overcharged state, the battery generates heat due to the change in the crystal structure of the positive electrode, decomposition of the electrolyte, release of oxygen due to the collapse of the positive electrode structure, etc. The internal temperature rises abnormally (so-called thermal runaway state), leading to heat generation and ignition. In general, as a countermeasure against such overcharge, when the temperature in the battery rises, the separator vacancy closes, so as to prevent the movement of lithium ions, so-called shutdown function to shield the current, Incorporates a mechanism to stop the charging current before thermal runaway and suppress or stop ignition or heat generation by providing a current interruption mechanism with an external circuit that detects the increase in internal pressure due to gas generation in the battery due to overcharge. It is. However, since it is not possible to ensure safety only by the above measures, various efforts have been made.

In Patent Documents 1 and 2, the increase in the internal pressure of the battery during overcharge is promoted by adding a material that decomposes at a potential that causes overcharge, such as Li ₂ CO ₃ or biphenyl, and generates gas in the electrolyte, Thermal runaway is prevented by operating a current interrupt mechanism that starts at a predetermined pressure at an early stage.

  In Patent Document 3, the thermal expansion microcapsule expands due to the heat generated by overcharging by including the thermal expansion microcapsule in the electrode or in the layer provided between the electrode and the current collector. Current is cut off by causing an increase, and thermal runaway is suppressed.

Japanese Patent No. 3061759 Japanese Patent No. 3575735 Japanese Patent No. 4727021

  However, in the method of incorporating a potential decomposable material in the electrolyte solution as shown in Patent Documents 1 and 2, the potential decomposable material dispersed in the electrolyte solution hinders lithium ion movement associated with charge / discharge of the battery, Battery capacity maintenance characteristics when current flows (hereinafter referred to as current load characteristics) and battery capacity maintenance characteristics when charging and discharging are repeated (hereinafter referred to as charge / discharge cycle characteristics) are degraded. In addition, when the internal pressure rises due to gas generation accompanying decomposition, the container may not withstand the internal pressure rise, or the internal gas may be deliberately released outside the container in order to suppress an excessive rise in internal pressure. The flammable electrolyte solvent is also released, and the danger may increase depending on the external environment.

  On the other hand, in the case where thermal expandable microcapsules that function by heat are contained in the electrode as shown in Patent Document 3 or in the layer provided between the electrode and the current collector, the basic configuration of the electrode is as described above. Since the capacity of the battery is determined by the amount of active material, the active material ratio in the electrode layer will be reduced if materials other than the basic composition are mixed in the electrode layer. It decreases, and the subject that the battery capacity per electrode volume decreases will remain.

  The present invention has been made in order to solve the above-mentioned problems, and lithium ions that can improve safety during overcharge without impairing various battery characteristics such as current load characteristics and charge / discharge cycle characteristics. It aims at providing the electrode for secondary batteries.

The present invention according to claim 1 is an electrode for a lithium ion secondary battery comprising a heat-shrinkable layer having conductivity and an active material layer on a current collector,
The electrode for a lithium ion secondary battery, wherein the heat shrinkable layer is made of a heat shrinkable resin.

  According to a second aspect of the present invention, in the electrode for a lithium ion secondary battery according to the first aspect, the heat-shrinkable resin is made of a polyolefin resin.

  The present invention according to claim 3 is the electrode for a lithium ion secondary battery according to claim 2, wherein the polyolefin resin is polypropylene or polyethylene.

  According to a fourth aspect of the present invention, in the lithium ion secondary battery according to any one of the first to third aspects, the heat shrinkage start temperature of the heat shrinkable layer is in the range of 130 ° C to 160 ° C. Electrode.

  According to a fifth aspect of the present invention, the heat shrinkable layer includes a material selected from one or more of a metal, a metal compound, amorphous carbon, crystalline carbon, and a conductive polymer as a conductive material. It is an electrode for lithium ion secondary batteries in any one of Claims 1-4 characterized by these.

  According to the first to third aspects of the present invention, an electrode for a lithium ion secondary battery comprising a conductive heat shrink layer and an active material layer on a current collector, wherein the heat shrink layer is a heat shrink resin. Therefore, the heat shrinkable layer contracts due to heat generated by overcharging. As a result, peeling occurs at the interface between the current collector and the heat-shrinkable layer, and the charging current is shielded by the increase in battery resistance, and the progress of overcharging can be suppressed.

  In addition, the polyolefin-based resin, particularly polypropylene or polyethylene, is used as a separator of a lithium ion secondary battery (hereinafter simply referred to as a secondary battery) as the heat-shrinkable layer. Is an excellent material with a proven track record and can further promote the above effects.

  According to claim 4, when the heat shrink start temperature of the heat shrink layer is in the range of 130 ° C. to 160 ° C., the solvent contained in the active material layer forming composition in the subsequent step of forming the active material layer The heat shrinkable layer can be prevented from shrinking at the temperature to be removed (usually 80 to 120 ° C.). In general, the thermal runaway of the secondary battery often occurs around 140 ° C., and it is possible to prevent an important matter by causing the thermal contraction appropriately even during the thermal runaway.

  According to claim 5, the heat shrinkable layer includes a material selected from one or more of a metal, a metal compound, amorphous carbon, crystalline carbon, and a conductive polymer as a conductive material. The electric body and the active material layer can be electrically connected, and the progress of overcharge can be suppressed only at the time of abnormality that impairs the function as an electrode.

  As described above, in the secondary battery using the lithium ion secondary battery electrode according to the present invention, the heat contraction layer contracts due to heat generated by overcharging, and as a result, the current collector, the heat contraction layer, As a result, the battery resistance increases, the charging current is shielded, and the progress of overcharging can be suppressed.

The schematic sectional drawing which shows one Embodiment of the electrode for lithium ion secondary batteries which concerns on this invention. The schematic sectional drawing which shows one Embodiment of the manufacturing method of the heat contraction layer of FIG.

  Hereinafter, an embodiment of the present invention will be described with reference to FIGS.

  As shown in FIG. 1, the present invention is an electrode 10 for a lithium ion secondary battery comprising a heat-shrinkable layer 2 having conductivity and an active material layer 3 on a current collector 1, wherein the heat-shrinkable layer 2 is heated. It consists of a shrinkable resin.

  As the current collector 1 according to the present invention, a conductive material can be used, and examples thereof include aluminum, stainless steel, and copper. Among these, foil-like aluminum and copper (hereinafter referred to as aluminum foil and copper foil) are preferable.

  Aluminum foil or copper foil is a material generally used for the positive electrode or negative electrode current collector of a lithium ion secondary battery. If it is cheap and aluminum foil is not alloyed with lithium at the potential of the positive electrode, it is made of copper foil. If present, it is not alloyed with lithium at the potential of a commonly used graphite negative electrode. In a material that is alloyed with lithium at the potential of each electrode, the current collector becomes brittle with charge and discharge in which lithium ions are inserted and released, so that the charge and discharge cycle characteristics of the battery are significantly reduced.

  Further, the thickness of the current collector 1 is not particularly limited, but it is preferable that the current collector 1 is thinner as long as the strength that can withstand the load in the battery manufacturing process is maintained and the highest energy density is obtained with respect to the weight and volume of the battery. preferable. The thickness of an aluminum foil or copper foil used for a current collector of a general lithium ion secondary battery is about 8 μm to 20 μm.

  Next, the heat shrinkable layer 2 that is a feature of the present invention will be described below.

  The heat-shrinkable layer 2 is formed on the current collector 1 and contracts due to heat generated during overcharge when it is incorporated in a battery as an electrode. That is, peeling occurs at the interface between the current collector 1 and the heat shrinkable layer 2 due to the heat generation, and the charging current is shielded by the increase in battery resistance, thereby suppressing the progress of overcharging.

  The heat-shrinkable layer 2 according to the present invention is made of a heat-shrinkable resin containing a conductive material, and the heat-shrinkable resin is preferably a resin that has a property of shrinking by heating, such as a polyolefin resin. Among them, polyethylene and polypropylene are materials that are also used as separators for lithium ion secondary batteries, and are more suitable in terms of electrolytic solution resistance and potential resistance. The thickness of the heat-shrinkable layer 2 depends on the heat-shrinkable resin and the conductive material, but it is preferable that the heat-shrinkable layer 2 is thinner as long as the heat-shrinkable function is maintained in order to reduce the resistance during normal operation in the battery.

  The heat shrink start temperature of the heat shrink layer 2 is preferably in the temperature range of 130 ° C to 160 ° C. Generally, the environmental temperature in which lithium ion secondary batteries are used is assumed to be about -20 ° C to 70 ° C. Therefore, when the thermal shrinkage start temperature is 80 ° C or less, thermal shrinkage starts in a normal battery usage environment. Therefore, the battery performance may be significantly deteriorated due to the increase in resistance due to the interface peeling between the current collector and the heat-shrinkable layer.

  The active material layer 3 is formed by applying a composition for forming an active material layer comprising an active material or a solvent onto the heat-shrinkable layer 2 on a current collector, followed by a drying step and the like. It is formed by removing the unnecessary solvent. In general, the drying process is often performed in a temperature range of 80 ° C to 120 ° C. At this time, if the heat shrinkage start temperature is lower than 130 ° C., the heat shrink layer functions in the electrode layer forming step, which hinders normal battery operation.

  Furthermore, although the thermal runaway of the battery depends on the positive electrode active material species and the negative electrode active material species selected, in general, it often occurs around 140 ° C. Usually, a separator used for a lithium ion battery has a function of closing a vacancy in the separator at a temperature of 120 ° C. or more and shielding the movement of lithium ions (referred to as a shutdown function). Therefore, the upper limit of the heat shrinkage starting temperature of the heat shrinkable layer of the present invention is preferably 160 ° C.

  The conductive material according to the present invention is not particularly limited as long as it is a material that is in contact with the current collector 1 and can be electrically connected to the active material layer 3 formed on the heat shrinkable layer 2. I can do it. For example, a metal, a metal compound, amorphous carbon, crystalline carbon, a conductive polymer, or a combination thereof may be used. Specifically, it is preferable to mix with the heat-shrinkable resin and uniformly disperse it. The amount to be mixed is not particularly limited as long as the current collector 1 and the active material layer 3 formed thereafter can be electrically connected and the electrical characteristics are not impaired.

  As a method for forming the heat-shrinkable layer 2 on the current collector 1, for example, using an extruder 20 as shown in FIG. 2, a heat-shrinkable resin containing a conductive material is applied to the cooling roll 22. The heat-shrinkable layer 2 can be formed by extruding onto the current collector 1 that conveys.

  By forming an active material layer on the heat-shrinkable layer 2 formed as described above, the secondary battery electrode of the present invention can be produced.

  Note that the active material layer forming composition for forming the active material layer 3 described above includes an active material, a conductive additive, a binder (binder resin), a solvent, and the like.

The active material is not particularly limited, and a commonly used material can be appropriately selected. For example, LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiFePO ₄
, LiMnPO ₄ , LiCo _x M _(1-x) O ₂ (M: fiber metal such as Mn, Ni, 0 <x <1), LiNi _x Co _y Mn _(1-xy) O ₂ (0 <x <1, 0 <y <1, 0 <x + y <1) and the like, and can be appropriately selected according to the voltage and capacity characteristics required for the battery. In particular, a layered compound such as LiCoO ₂ , LiNiO ₂ , LiNi _x Co _y Mn _(1-xy) O ₂ (0 <x <1, 0 <y <1, 0 <x + y <1) is used as an active material. When used, it is known that thermal runaway tends to occur during overcharge.

  Moreover, as a conductive support agent, highly conductive materials, such as an amorphous carbon material, a crystalline carbon material, a metal particle, a metal compound particle, can be selected suitably, for example.

  Furthermore, as the binder used in the electrode layer, for example, polyvinylidene fluoride, styrene butadiene rubber, polyimide, polytetrafluoroethylene, or the like can be selected.

  Hereinafter, the present invention will be described by way of examples.

<Example 1>
A heat shrink layer made of polypropylene and acetylene black was formed on the aluminum foil having a thickness of 20 μm as a current collector by the following method.

(Formation of heat shrink layer)
Polypropylene and acetylene black are uniformly mixed at a weight ratio of 70:30 using a uniaxial stirrer, and then extruded onto the aluminum foil at a temperature of 240 ° C. and simultaneously cooled using an extruder. A heat-shrinkable layer having a thickness of 2 μm was formed.

(Preparation of positive electrode)
Separately, a mixture of LiCoO ₂ as an active material, acetylene black as a conductive additive, polyvinylidene fluoride as a binder, and n-methyl-2-pyrrolidone as a solvent is stirred for 30 minutes with a biaxial kneading stirrer for 30 minutes. A forming composition was prepared. The weight ratio of LiCoO _2, acetylene black, and polyvinylidene fluoride was 90: 5: 5.

Next, the active material layer-forming composition was applied onto the heat-shrinkable layer of the current collector so as to be 13 mg / cm ² using an applicator. Thereafter, in order to completely remove the solvent, a drying process was performed at 100 ° C. for 1 hour, and further, a drying process was performed again at 100 ° C. for 12 hours in a vacuum environment to produce a positive electrode.

(Preparation of negative electrode)
A composition for forming an active material layer for a negative electrode comprising scaly graphite, acetylene black as a conductive additive, styrene butadiene rubber as a binder, and carboxymethyl cellulose for adjusting viscosity is used on a copper foil as a current collector. Thus, a negative electrode was produced by the following method.

  The negative electrode active material layer forming composition was prepared by adding water and viscosity-adjusting carboxymethylcellulose at a weight ratio of scaly graphite, acetylene black, and styrene butadiene rubber of 96 to 3 to 1, and using a uniaxial kneading stirrer. Stir for hours to adjust. The amount of carboxymethyl cellulose added was 1 part by weight with respect to 100 parts by weight of the total composition.

Next, the composition was coated on a copper foil using an applicator so as to be 6 mg / cm ² . Then, in order to remove a solvent completely, the drying process was implemented at 100 degreeC for 1 hour, and also the drying process was implemented again at 100 degreeC for 12 hours in a vacuum environment, and the negative electrode was produced.

(Production of secondary battery)
A solvent in which ethylene carbonate and diethyl carbonate are mixed at a ratio of 1 to 1 in a weight ratio of 1 mol / L lithium hexafluorophosphate using the positive electrode and the negative electrode prepared above. A laminate type secondary battery using was manufactured. The capacity of the secondary battery was designed to be 500 mAh by stacking the positive electrode and the negative electrode.

<Comparative Example 1>
A secondary battery was fabricated in the same manner as in Example 1 except that the heat shrink layer was not formed on the current collector.

<Evaluation and method>
Using the secondary batteries produced in Example 1 and Comparative Example 1, the appearance, current load characteristics, and charge / discharge cycle characteristics during overcharging were evaluated by the following methods. The results are shown in Table 1 below.

(Appearance evaluation during overcharge)
Under an environment of 25 ° C., the battery was charged to 12 V at a charging current of 1500 mA, and the state of the battery at that time was visually observed.

(Evaluation of current load characteristics)
In an environment of 25 ° C., the battery is charged to 4.1 V at a charging current of 250 mA, then discharged to 3 V at a discharging current of 250 mA, and further charged to 4.1 V at a charging current of 250 mA, and then discharged to 3 V at a discharging current of 2000 mA. did. The ratio of the battery capacity obtained when discharged at a discharge current of 2000 mA to the battery capacity obtained when discharged at a discharge current of 250 mA in this test was calculated as the load capacity retention rate.

(Evaluation of charge / discharge cycle characteristics)
In an environment of 25 ° C., after charging to 4.1 V at a charging current of 500 mA, charging / discharging to discharge to 3 V at a discharging current of 500 mA is defined as one cycle. The discharge capacity at the cycle was calculated as the capacity retention rate.

<Comparison result>
The secondary battery comprising the product of the present invention obtained in Example 1 has both good current load characteristics and charge / discharge cycle characteristics, and no abnormalities are observed in visual observation during overcharge, and excellent results are obtained. It was. On the other hand, the secondary battery obtained in Comparative Example 1 was equivalent to Example 1 in both current load characteristics and charge / discharge cycle characteristics, but generation of white smoke was confirmed by visual observation during overcharge. From these results, it was shown that the electrode for lithium ion secondary batteries of this invention can improve the safety | security at the time of overcharge, without impairing charging / discharging performance.

  The configuration of the electrode of the present invention can be applied not only to the field of lithium ion secondary batteries but also to all fields related to energy devices that require safety.

DESCRIPTION OF SYMBOLS 1 ... Current collector 2 ... Heat shrink layer 3 ... Active material layer 10 ... Electrode 20 for lithium ion secondary batteries ... Extruder 21 ... Unwinding part 22 ... Cooling roll 23・ Winding part

Claims (5)

An electrode for a lithium ion secondary battery comprising an electrically conductive heat shrink layer and an active material layer on a current collector,
The electrode for a lithium ion secondary battery, wherein the heat shrinkable layer is made of a heat shrinkable resin.   The electrode for a lithium ion secondary battery according to claim 1, wherein the heat-shrinkable resin is made of a polyolefin-based resin.   The electrode for a lithium ion secondary battery according to claim 2, wherein the polyolefin resin is polypropylene or polyethylene.   The electrode for a lithium ion secondary battery according to any one of claims 1 to 3, wherein a heat shrink start temperature of the heat shrink layer is in a range of 130 ° C to 160 ° C.   The heat shrink layer includes a material selected from one or more of a metal, a metal compound, amorphous carbon, crystalline carbon, and a conductive polymer as the conductive material. The electrode for lithium ion secondary batteries in any one.