JP3300168B2 - Negative electrode for non-aqueous electrolyte secondary battery, method for producing the same, and non-aqueous electrolyte secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a non-aqueous electrolyte secondary battery,
In particular, it relates to improvement of a negative electrode integrated with an electrolyte.

[0002]

Nowadays, propylene carbonate, .gamma.-butyrolactone, dimethoxyethane, tetrahydrofuran, in organic solvent dioxolane, LiClO _4, Li
A non-aqueous electrolyte battery in which an electrolyte obtained by dissolving a solute such as BF ₄ , LiAsF ₆ , LiPF ₆ , or LiCF ₃ SO ₃ and a negative electrode using an alkali metal such as lithium as an active material has a high energy density Therefore, it has been widely used for small electronic devices such as electronic watches and cameras. One of the issues that make this type of non-aqueous electrolyte battery rechargeable is that the form of alkali metal deposited on the negative electrode during the charging process is dendritic, fibril-like, and needle-like.
What is called a dendrite. If this dendrite grows remarkably, not only the danger of internal short circuit between the negative electrode and the positive electrode, but also the risk of ignition will increase, and even if the dendrite is dissolved in the subsequent discharge process, local dissolution of the dendrite will proceed, and part of it will be electrically All dendrites cannot be melted out due to liberation. That is, the amount of discharge (dissolution) relative to the amount of charge (precipitation) is reduced, the charge / discharge efficiency is reduced, and the cycle life is shortened.

[0003] As a method of solving such a problem,
It has been proposed to control dendrites by using solid polymer electrolytes instead of electrolytes (FastIon Transport in Solids, North-Holland, New
York, 1979, p. 131). Here, the polymer electrolyte is
Refers to a mixture of a polymer having a polar atom such as oxygen in its molecular chain (for example, polyethylene oxide) and an alkali metal salt that dissolves and dissociates in the polymer. The mixture contains a solvent such as propylene carbonate. You may go out.

[0004]

When the above-mentioned polymer electrolyte is used in a non-aqueous electrolyte secondary battery, the internal resistance of the battery gradually increases with the charge / discharge cycle, and when the electrolyte is used. There was a problem that the cycle life was shorter than that. This is for the following reason. That is, immediately after the battery is assembled or at the beginning of the charge / discharge cycle, the surface of the negative electrode using an alkali metal such as lithium as an active material is relatively flat, and the adhesion to the elastic polymer electrolyte is good. The interfacial resistance of the polymer electrolyte is low.
However, as the charge / discharge cycle progresses, the surface of the negative electrode becomes uneven due to local dendrite growth,
The polymer electrolyte is lifted around each of the dendrite growth points, and the negative electrode and the polymer electrolyte are separated. As a result, the interface resistance between the negative electrode and the polymer electrolyte increases, making it difficult to charge and discharge the battery. The present invention is to eliminate such a conventional disadvantage, and even if the charge and discharge cycle is repeated, the interface resistance between the negative electrode and the polymer electrolyte does not increase, and the generation of dendrites on the negative electrode is suppressed. An object is to provide a highly reliable nonaqueous electrolyte secondary battery by obtaining an integrated negative electrode.

[0005]

This onset Ming nonaqueous negative electrode for electrolyte secondary battery SUMMARY OF THE INVENTION may, ultraviolet light or heat surface area is closely integrated with the surface of 10000 cm ² / g or more alkali metal foils and the alkali metal foil Having an alkali ion conductive polymer electrolyte cured by Further, the method for producing a negative electrode for a non-aqueous electrolyte secondary battery according to the present invention comprises the steps of electrochemically dissolving an electrode made of an alkali metal foil , and forming an uneven surface having a surface area of 10,000 cm ² / g or more on the electrode surface. Then , a polymer electrolyte material that is cured by ultraviolet light or heat is cured on the electrode surface. Further, the non-aqueous electrolyte secondary battery of the present invention includes the negative electrode firmly integrated with the alkali ion-conductive polymer electrolyte as described above. Here, the definition of integration refers to a case where 90% or more of the negative electrode surface area is covered (wetted) by the polymer electrolyte, and the strong integration means that the polymer electrolyte is substantially separated from the negative electrode surface by the electrolyte. Can not be peeled off without traces.

[0006]

When the surface of the negative electrode containing an alkali metal as an active material has irregularities, the material (solution) for the polymer electrolyte before curing easily penetrates into the concave portions of the electrode, and the entire negative electrode surface is used for the polymer electrolyte. Wet without gaps with material. Thereafter, if the polymer electrolyte material is cured by polymerization or the like,
An integrated product of a strong polymer electrolyte and a negative electrode can be obtained. Here, the reason why the polymer electrolyte and the negative electrode firmly adhere to each other is not clear, but is presumed as follows. First, when irregularities are formed on the negative electrode surface, the surface area of the negative electrode increases. For this reason, by increasing the physical attraction between the polymer electrolyte and the negative electrode, a strong integration can be obtained. Second, polymers significantly change their bulk density before and after curing. In particular, the ion conductive polymer electrolyte material hardens to reduce the bulk density, that is, increase the volume. Therefore, the material for the polymer electrolyte that has permeated into the concave portion tends to expand in the concave portion due to curing, and as a result, a material such as a key-keyhole relationship formed by the convex portion of the polymer electrolyte and the concave portion of the negative electrode is formed. Strong integration between the polymer electrolyte and the negative electrode is obtained. The above action is obtained only when the surface of the negative electrode using an alkali metal as an active material has fine irregularities of micro to sub-micron or less, and the surface area of the negative electrode is 10,000 cm.
² / g or more. This is because the physical bonding force between the polymer electrolyte and the negative electrode is small, so that it is necessary to increase the surface area by an order of magnitude to increase the bonding force as a whole.

[0007]

Embodiments of the present invention will be described below. Note that all the examples were performed in an argon gas atmosphere. Although lithium was used as the alkali metal of the negative electrode active material, similar results can be obtained by using other alkali metals or alloys with alkali metals. Example 1 In order to produce lithium electrode foils having various surface areas, an alternating current was passed in a propylene carbonate solution in which LiClO ₄ was dissolved at a concentration of 1.0 mol / l to dissolve the lithium surface. A lithium foil having a thickness of 300 μm was used, the current density was ² mA / cm ² , and the anode time was 1
Minute and cathode time were set to 30 seconds, and anode dissolution and cathode deposition were defined as one cycle. By changing the number of cycles, a lithium electrode foil having a surface area of 130 to 58000 cm ² / g was obtained. Here, 130 cm ² / g is the surface area in the case of untreated. The polymer electrolyte material was prepared as follows. A polyethylene glycol diacrylate having an average weight molecular weight of 8000 and a propylene carbonate solution of LiClO ₄ (concentration: 1 M) were mixed at a weight ratio of 4 /
6 were mixed. To this, 100 ppm of an ultraviolet curing initiator (Irgacure 651 manufactured by Ciba Geigy) was added to obtain a polymer electrolyte material.

[0008] A polymer electrolyte material is poured onto the lithium electrode foil having the uneven surface prepared as described above, and the surface of the polymer electrolyte material is irradiated with ultraviolet rays at 45 mW / cm ² for 3 minutes to irradiate the polymer electrolyte material. As a result, an integrated polymer electrolyte / lithium electrode foil having a thickness of about 600 μm in total of the lithium electrode foil and the polymer electrolyte was obtained. Observation with an optical microscope of all of the polymer electrolyte / lithium electrode foil integrated bodies produced in this way revealed that there were no bubbles between the polymer electrolyte and the lithium electrode foil, and that the surface of the lithium electrode foil was wet. confirmed.

Next, for each of the polymer electrolyte / lithium electrode foil integrated material, when the polymer electrolyte was to be peeled by sandwiching the polymer electrolyte portion with tweezers, the surface area of the lithium surface not electrochemically dissolved was 13%.
In the case of 0 cm ² / g, the polymer electrolyte was easily peeled off as a film of about 300 μm. On the other hand, in the case of a lithium electrode foil having a surface area of 58000 cm ² / g, the polymer electrolyte is not easily peeled off. And the polymer electrolyte could not be separated from the lithium surface without leaving any trace of the polymer electrolyte. The same operation was performed on lithium foils having various surface areas, and it was examined whether the polymer electrolyte could be peeled off from the lithium surface without leaving any trace. The result is shown in FIG. The surface area up to about 8000 cm ² / g was able to be peeled from the lithium surface without leaving a trace of the polymer electrolyte,
At 0 to 10,000 cm ² / g, peeling was possible without leaving any trace, and at 10,000 cm ² / g or more, no peeling was possible. From the above, the integrated product of the strong polymer electrolyte and the lithium electrode foil has a surface area of 10,000 cm ^2.
/ G or more.

Embodiment 2 In this embodiment, it is shown that a battery in which an electrode having a high surface area and a polymer electrolyte are firmly integrated can be easily manufactured without greatly changing the conventional battery assembly procedure. In place of the ultraviolet curing initiator added to the polymer electrolyte material used in Example 1, a thermosetting initiator benzoyl peroxide was used. A flat battery as shown in FIG. 2 was constructed using the polymer electrolyte material thus prepared. Hereinafter, description will be given based on FIG. The positive electrode 1 was prepared by mixing LiMn ₂ O ₄ powder, carbon black and polytetrafluoroethylene resin powder, and pressing the expanded metal current collector 2 made of titanium into a positive electrode can 3 by spot welding. The electrolyte material was vacuum impregnated. The negative electrode 4 was obtained by pressing a lithium sheet punched into a disc shape onto a sealing plate 6 to which a nickel expanded metal 5 was spot-welded. A porous battery made of polypropylene was used as the separator 7, and after pouring the above-mentioned polymer electrolyte material, the positive electrode can 3 and the sealing plate 6 were combined via the gasket 8 to form a flat battery. next,
The battery assembled in this manner was heated at 25 ° C.
After repeating a charge / discharge cycle three times under the conditions of a current density of 0 mA / cm ² , a discharge lower limit voltage of 2.0 V, and a charge upper limit voltage of 3.5 V, the battery is warmed to 80 ° C., and the polymer electrolyte material is cured in the battery. This was used as a polymer electrolyte. When some of the batteries were decomposed before and after the material for the polymer electrolyte was cured, the surface area of the lithium anode was 17000 cm ² /
g, and it was confirmed that the polymer electrolyte and the lithium anode were integrated as in Example 1. Comparative Example 1 A battery was assembled in the same manner as in Example 2 except that the separator was impregnated with a material for a polymer electrolyte and a separator-polymer electrolyte integrated body previously cured at 80 ° C. was bonded to a lithium anode.

The batteries of Example 2 and Comparative Example 1 configured as described above were subjected to a current density of 2.0 mA / cm ² , a discharge lower limit voltage of 2.0 V, and a charge upper limit voltage of 3.5 at 25 ° C.
The charge / discharge cycle was repeated under the condition of V, the discharge capacity at each cycle number was determined, and the point at which the discharge capacity became half of the capacity at the first cycle was defined as the cycle life. FIG.
It is a plot of the discharge capacity in each cycle. FIG. 3 shows that the battery of Example 2 had a significantly improved cycle life than the battery of Comparative Example 1. This is because, in the battery of the present invention, the generation of dendrites on the negative electrode during charging is suppressed, and the charge and discharge efficiency of the negative electrode is improved,
As a result, the cycle life of the negative electrode was extended.
The cycle life of the battery of the comparative example did not increase because the previously cured polymer electrolyte was bonded to the lithium anode, so that their adhesion strength was small, and the interface between the polymer electrolyte and the lithium anode was dendritic due to the charge / discharge cycle. This is because they are peeled off by the occurrence and the resistance inside the battery increases.

Example 3 Example 2 was repeated using the material for a polymer electrolyte to which the thermosetting initiator used in Example 2 was added.
A flat battery similar to the above was constructed. The battery was subjected to three charge / discharge cycles at 25 ° C. under the conditions of a current density of 2.0 mA / cm ^2, a lower discharge voltage of 2.0 V, and a upper charge voltage of 3.5 V, and then the battery was warmed to 80 ° C. The polymer electrolyte material was cured in the battery to obtain a polymer electrolyte. [Comparative Example 2] The surface of a lithium negative electrode was appropriately pressed with a 60-mesh net made of stainless steel to have a surface area of 18000c.
The polymer electrolyte material was cured without forming a concave / convex shape of m ² / g without performing a charge / discharge cycle of the battery to integrate the polymer electrolyte and the negative electrode.

The batteries of Example 3 and Comparative Example 2 configured as described above were subjected to a current density of 2.0 mA / cm ² , a discharge lower limit voltage of 2.0 V, and a charge upper limit voltage of 3.5 at 25 ° C.
The charge / discharge cycle was repeated under the condition of V, and the discharge capacity at each cycle number was determined. FIG. 4 plots the discharge capacity of the battery in each cycle. From FIG.
Although the cycle characteristics of the battery of Comparative Example 2 were significantly improved as compared with Comparative Example 1 in which the polymer electrolyte was bonded, the cycle characteristics were inferior to the battery of Example 3. This is because, in the battery of Example, the unevenness on the negative electrode surface was formed by electrochemical dissolution, and the distribution and depth of the unevenness were relatively uniform. This is because there is little separation at the interface between the anode and the negative electrode, and the generation of dendrites is suppressed. On the other hand, the unevenness on the surface of the negative electrode in the battery of Comparative Example 2 has a large variation and the adhesive strength at the interface between the polymer electrolyte and the negative electrode is sparse, so that the dendrite is generated at a low strength and the charge / discharge efficiency is reduced. is there.

[0014]

As described above, according to the present invention, the generation of dendrites on the negative electrode during charging is small and the charge / discharge efficiency is improved, so that a highly reliable nonaqueous electrolyte secondary battery having a long cycle life can be obtained. Can be In addition, after the battery is constructed, a preliminary charge / discharge cycle can be performed to obtain a negative electrode having irregularities with a predetermined surface area.If the material for the polymer electrolyte is cured at that point, the normal assembly procedure can be greatly reduced. A polymer electrolyte battery can be easily manufactured without any change.

[Brief description of the drawings]

FIG. 1 is a diagram showing the surface area of a negative electrode and the adhesion strength of a polymer electrolyte in an example of the present invention.

FIG. 2 is a longitudinal sectional view of a flat type battery used in an example of the present invention.

FIG. 3 is a diagram plotting the discharge capacities of the batteries of Example 2 and Comparative Example 1 in each cycle.

FIG. 4 is a diagram in which the discharge capacities of the batteries of Example 3 and Comparative Example 2 in each cycle are plotted.

[Explanation of symbols]

 Reference Signs List 1 positive electrode 2 positive electrode current collector 3 positive electrode can 4 negative electrode 5 negative electrode current collector 6 sealing plate 7 separator 8 gasket

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-223159 (JP, A) JP-A-62-296376 (JP, A) JP-A-2-295070 (JP, A) 507171 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 4/02 H01M 4/04 H01M 10/40

Claims (3)

(57) [Claims] 1. A non-aqueous electrolyte secondary battery comprising an alkali metal foil having a surface area of 10,000 cm ² / g or more and an alkali ion conductive polymer electrolyte which is tightly integrated with the surface of the alkali metal foil and cured by ultraviolet light or heat. Negative electrode. 2. An electrode made of an alkali metal foil is electrochemically dissolved to have a surface area of 10,000 cm on the electrode surface.
² / g or more irregularities are formed, and then UV or heat
A method for producing a negative electrode for a non-aqueous electrolyte secondary battery, comprising curing a material for a polymer electrolyte that cures on the electrode surface. 3. A positive electrode, and a nonaqueous electrolyte secondary battery having a negative electrode having a claim 1 A Rukariion conductive polymer electrolyte according.