JP6955660B2 - Power storage element - Google Patents

Description

The present invention relates to a power storage element (hereinafter, may be simply abbreviated as "power storage element") such as a lithium ion secondary battery and a lithium ion capacitor that utilize insertion / removal of lithium ions.

Conventionally, the negative electrode of a power storage element has an active material layer containing a particulate carbon-based active material such as graphite powder and a binder formed on the surface of a foil-like current collector such as copper foil or stainless steel. It is used. Here, as the binder, polyvinylidene fluoride, polytetrafluoroethylene, polyimide or the like is used.

Since the theoretical capacity of the negative electrode using the carbon-based active material is 372 mAh / g, a higher capacity active material is required. Therefore, as a next-generation active material that replaces carbon-based active materials, negative electrode active materials such as silicon-based active materials and tin-based active materials that exhibit a theoretical discharge capacity several times or more that of graphite by alloying with lithium (hereinafter, simply "alloy-based active materials"). Negative electrodes using "negative electrode active material") have been proposed.

However, since these alloy-based negative electrode active materials have a large volume change (expansion / contraction) due to charging / discharging, the active material becomes finer or desorbs from the current collector as the charging / discharging is repeated. There was a problem that the discharge capacity was significantly reduced.

As a method for improving this decrease in cycle characteristics, Patent Document 1 proposes a method of forming a negative electrode having a SiLix film on the outermost surface by a vapor deposition method on a current collector metal foil. Document 2 proposes a method of using silicon oxide particles having a particle internal Fe content of 10 to 1,000 ppm and a particle external Fe content of 30 ppm or less as a negative electrode active material.

JP-A-2009-43747 Japanese Unexamined Patent Publication No. 2013-258135

All of the negative electrode active materials described in the above patent documents focus on the surface of the negative electrode active material, and attempt to improve the deterioration of the cycle characteristics by increasing the activity of the surface, but repeatedly. The volume change of the negative electrode active material due to charging and discharging is large, and it is difficult to sufficiently suppress the decrease in discharge capacity due to this. Therefore, it is required to maintain a high discharge capacity even after repeated charging and discharging.

Therefore, in order to solve the above-mentioned problems, it is an object of the present invention to provide a power storage element capable of maintaining a high discharge capacity even after repeated charging and discharging, for example, when an alloy-based negative electrode active material is used.

The present inventor has arrived at the present invention as a result of repeated diligent research in order to solve the above-mentioned problems. That is, the gist of the present invention is as follows.

(1) The amount of lithium ions inserted into an alloy-based negative electrode active material having an average particle size of 1 μm or less is regulated within a range of 2 % or more and less than 20% of the theoretical capacity of the negative electrode active material, and the lithium ions insertion amount, 50~500mAh / g is in the range of the power storage element utilizing the intercalation and deintercalation of lithium ions.

The power storage element of the present invention can maintain a high discharge capacity even after repeated charging and discharging.

As the negative electrode of the power storage element of the present invention, for example, an active material layer containing a particulate negative electrode active material and a binder is preferably formed on the surface of a foil-like current collector such as copper foil or stainless steel. Can be used. Here, as the binder, known binders such as polyvinylidene fluoride, polytetrafluoroethylene, and polyimide can be used, but it is preferable to use polyimide. Further, it is preferable to provide a porous conductive adhesive layer on the current collector. These negative electrodes can be obtained, for example, by the method described in International Patent Publication No. WO2014 / 017506.

The type of negative electrode active material used is not limited, but an alloy-based negative electrode active material having a high theoretical capacity can be preferably used. Specific examples of the alloy-based negative electrode active material include silicon (theoretical capacity: 4210 mAh / g), silicon oxide (theoretical capacity: 2680 mAh / g), germanium (theoretical capacity: 1620 mAh / g), and tin (theoretical capacity: 994 mAh / g). And so on. As the shape of these negative electrode active materials, an indefinite shape, a spherical shape, a fibrous shape, or the like can be used, and there is no limitation. The average particle size on a volume basis is preferably 1 μm or less, and more preferably 0.5 μm or less. The reason will be described later.

The negative electrode active material and the binder are mixed in a solvent to form a coating liquid, which is applied to the current collector and dried to obtain a negative electrode having a negative electrode active material layer formed on the current collector. Can be done. Here, it is preferable to add a conductive auxiliary agent such as graphite or carbon black to the negative electrode active material coating liquid in the negative electrode active material layer.

In the power storage element of the present invention having the negative electrode obtained as described above, the amount of lithium ions inserted into the negative electrode active material is within the range of 1% or more and less than 30% of the theoretical capacity of the negative electrode active material. Preferably, it is regulated within the range of 2% or more and less than 20% so that the battery can be charged. In order to do so, when the power storage element of the present invention is used as, for example, a lithium ion secondary battery, the positive electrode is so that the amount of lithium ions that move from the positive electrode to the negative electrode during charging is within this range. The amount of lithium ions may be adjusted. When used as a lithium ion capacitor, the amount of lithium in this range may be inserted into the negative electrode in advance. By doing so, when the power storage element is used, the lithium ion insertion amount of the negative electrode is 1% or more of the theoretical capacity when the power storage element is fully charged (100% charged). , Can be within the range of less than 30%. As a result, it is possible to suppress the volume change (expansion) of the negative electrode active material in proportion to the amount of inserted lithium ions.

Further, as described above, by reducing the particle size of the negative electrode active material to 1 μm or less and increasing the surface area thereof, the ratio of lithium ions adsorbed on the surface of the negative electrode active material can be further increased by charging. .. A film called SEI (Solid Electrolyte Interface) is formed on the surface of the negative electrode active material by charging and discharging. Lithium ions adsorbed on the surface of this film are electrochemically active, and by discharging, they are formed. It can be taken out as an electric current. Therefore, by increasing the surface area, the discharge capacity can be increased. Since the SEI film does not change in volume (expansion) due to the adsorption of lithium ions, the volume change (expansion) of the entire negative electrode active material due to the insertion of lithium is made smaller by increasing the surface area of the negative electrode active material. This makes it possible to significantly suppress a decrease in cycle characteristics due to expansion and contraction of the negative electrode active material. The "lithium ion insertion amount" referred to in the present invention means the total of the lithium ion insertion amount based on the alloying reaction and the like and the lithium ion insertion amount adsorbed on the surface.

The power storage element of the present invention regulates the amount of lithium ions inserted within the range of 1% or more and less than 30% of the theoretical capacity of the negative electrode active material so that it can be charged. It is preferably 10 mAh / g to 1000 mAh / g, and more preferably 50 to 500 mAh / g, based on the mass of the negative electrode active material.

Further, the current density in the power storage element of the present invention is preferably 10 mA / g to 2000 mA / g, more preferably 50 to 500 mAh / g, based on the mass of the negative electrode active material.

Hereinafter, examples of the present invention will be described in detail, but the present invention is not limited to these examples.

<Example 1>
Based on the description of Example 1 of International Patent Publication No. WO2014 / 017506, a negative electrode containing silicon (theoretical discharge capacity: 4400 mAh / g) as an active material was obtained. That is, a conductive particle dispersion (composition ratio: graphite 82% by mass, polyamide-imide 18% by mass) was applied to one surface of an electrolytic copper foil (F2-WS manufactured by Furukawa Denki Kogyo Co., Ltd.) having a thickness of 18 μm using a bar coater. After uniformly applying the mixture, the mixture was dried at 130 ° C. for 10 minutes to obtain a conductive coating film. The coating amount of the graphite dispersion was adjusted so that the thickness of the obtained conductive adhesive layer was 3 to 4 μm. Next, a silicon dispersion (composition ratio: 64% by mass of silicon, 20% by mass of polyimide, 16% by mass of graphite) was uniformly applied to the surface of the conductive coating film in a sheet-like manner using a bar coater, and 10 at 130 ° C. It was dried for a minute to obtain an active material coating film. The coating amount of the silicon dispersion was adjusted so that the thickness of the obtained active material layer was 40 to 50 μm. In this way, a laminate obtained by laminating the electrolytic copper foil, the conductive coating film, and the active material coating film in this order was obtained. The average particle size of the silicon used here was 0.5 μm. Next, the obtained laminate was heated from 100 ° C. to 350 ° C. over 2 hours in a nitrogen gas atmosphere, and then heat-treated at 350 ° C. for 1 hour. By this heat treatment, the polyamic acid in the active material coating film was converted into polyimide. In this way, a negative electrode formed by laminating the electrolytic copper foil, the conductive adhesive layer, and the active material layer in this order was obtained.
Using the obtained sheet-shaped negative electrode, a bipolar pouch type cell (laminate cell) was produced as a test cell for measuring the discharge capacity of the negative electrode by the following method. The obtained sheet-shaped negative electrode was cut into a rectangular shape of 10 mm × 40 mm and coated with a fusion film leaving an active material area of 10 mm × 10 mm. As a counter electrode, a lithium plate having a thickness of 1 mm was cut into a rectangular shape having a thickness of 30 mm × 40 mm, folded in half into a nickel lead (5 mm × 50 mm) having a thickness of 0.5 mm, and pressure-bonded. Only the negative electrode was placed in a bag-shaped separator (30 mm × 20 mm) and then faced with the counter electrode to obtain an electrode group. A rectangular polypropylene resin porous film (thickness 25 μm) was used as the separator. This electrode group was covered with a set of two rectangular aluminum laminate films (50 mm × 40 mm), the three sides thereof were sealed, and then 1 mL of the electrolytic solution was injected into the bag-shaped aluminum laminate film. As the electrolytic solution, a mixture of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a mixed solvent having a volume ratio of 1: 1: 1 and LiPF ₆ dissolved at a concentration of 1 mol / L was used. Then, the remaining one side was sealed, and the inside of the bag-shaped aluminum laminated film was sealed. Further, when sealing the inside of the bag-shaped aluminum laminated film, one end of the negative electrode and the nickel lead was extended outward to form a terminal. In this way, a test cell was obtained. All of these operations were performed in a glove box with an argon atmosphere.
Next, using the obtained test cell, charging / discharging was repeated under the charging / discharging conditions of measurement temperature: 30 ° C., charge amount: 500 mA / g, charge current density and discharge current density: 100 mA / g, and the 80th time. The discharge capacity was calculated.
The charge amount of 500 mA / g corresponds to 11.9% of the theoretical capacity of silicon (4210 mA / g). As a result of the measurement, it was found that the 80th discharge capacity was 495.2 mA / g-silicon, and 99% or more of the charged electricity amount was discharged. From this, it can be seen that by limiting the charge amount (lithium ion insertion amount) to 500 mA / g, the volume change of silicon due to charge / discharge is significantly suppressed, and good cycle characteristics can be ensured. Further, this capacity is significantly higher than the theoretical capacity of graphite (372 mAh / g).

As described above, the power storage element of the present invention in which the lithium ion insertion amount (charge amount) is regulated within a specific range has significantly improved cycle characteristics.

Claims (1)

The amount of lithium ions inserted into an alloy-based negative electrode active material having an average particle size of 1 μm or less is regulated within the range of 2 % or more and less than 20 % of the theoretical capacity of the negative electrode active material, and the amount of lithium ions inserted is limited. , in the range of 50~500mAh / g, the power storage element utilizing the intercalation and deintercalation of lithium ions.