JP2005011821A - Lithium secondary battery negative electrode member and lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a negative electrode member for a lithium secondary battery, having improved cycle characteristics and safety by suppressing the dendrite growth, caused through the reaction of a lithium metal and organic electrolyte solution during charge/discharge cycles. <P>SOLUTION: The negative electrode member 5A for a lithium secondary battery is formed by forming a polypropylene film 7 on a substrate of a copper foil 6 as an electric insulation layer, then forming a lithium metal film 3 on the copper foil 6 for conducting electricity, and forming an inorganic solid electrolytic membrane 4 thereon. A substrate of an electric insulating body can be used instead of the copper foil 6, to allow the lithium metal film 3 to also be used as a collecting body. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a negative electrode member of a lithium secondary battery having high safety, high capacity and excellent cycle characteristics, and a lithium secondary battery using the same.

  For the purpose of improving the volume and weight capacity density of a lithium ion secondary battery, a method of accumulating in a negative electrode in the state of lithium metal is used instead of a conventional method using intercalation of lithium ions into graphite. Although this method has been studied, this method involves dendritic growth in which lithium metal reacts with the organic electrolyte, and lithium is deposited in the form of dendritic crystals during charge and discharge. In addition to shortening, there is an internal short circuit with the positive electrode, which may eventually lead to an explosion.

  As a method for suppressing this dendrite growth, it has been studied in the past to form a solid electrolyte film such as a polymer film, a fluoride film, a carbonate compound film, an oxide film, an oxynitride film, or a sulfide film on the surface of lithium metal. These films are disclosed in Patent Documents 1 to 4 below.

  For the purpose of increasing the battery capacity per unit volume and weight, the lithium metal layer needs to have a thickness of 20 μm or less, preferably about 5 μm. However, in this thickness region, the lithium self-supporting foil has low mechanical strength. Therefore, the lithium metal layer is deposited on the base material by vapor deposition such as vapor deposition method such as using a strong current collector such as copper foil as a base material and bonding the lithium foil on the base material. It is necessary to form.

  Conventionally, an electrical conductor such as a copper foil has been used for a base material of a negative electrode of a lithium ion secondary battery.

On the other hand, in the method of suppressing dendrite growth by forming a solid electrolyte film on lithium metal, the lithium metal layer and sulfide-based solid electrolyte, which are highly hydrolyzable, are partially degraded in the negative electrode fabrication process and handling process. It may occur, and it is assumed that the coating effect by the solid electrolyte membrane is not exhibited. When such a situation occurs, dendritic growth occurs due to destruction of the solid electrolyte membrane in the deteriorated portion, leading to a reduction in cycle life. In addition, when an electrically conductive material is used for the base material, electrons continue to be supplied to the negative electrode, so that there is a high possibility that charge and discharge concentrate at that portion. Furthermore, there is a risk of causing an internal short circuit with the positive electrode due to the progress of dendrite growth, and finally an explosion.
US 5,314,765 (see claim 1) US 6,025,094 (see claim 1 and 4) JP 2000-340257 (see claims 6 and 7) JP 2002-329524 A (refer to claims 1 and 9)

  This invention makes it a subject to eliminate such a malfunction and to improve the cycling characteristics and safety | security of the negative electrode member for lithium secondary batteries.

  The inventors solved the technical problem of dendrite intensive growth in the negative electrode member of a lithium secondary battery in which a lithium metal film and a solid electrolyte film were formed on the base material by using an electrical insulator as the base material. I found out that In particular, the use of an organic polymer material as the substrate can enhance the effect of suppressing dendrite growth.

  Further, the same problem can be solved even with a structure in which an electrical insulator layer is provided on a metal substrate and this is used as a substrate. The metal substrate may be any of copper, iron, stainless steel, nickel, and aluminum. In addition, the electrical insulator layer may be formed by coating an organic polymer material. In this case, since the base portion of the base material is a metal foil, the mechanical strength of the negative electrode must be sufficiently ensured. Can do.

  Polyvinyl such as polyethylene and polypropylene is usually used as the organic polymer material, but polyimide, polyamide, polyester, polyether, polyurethane, or polycarbonate may be used, and even if these are used, the object of the invention is achieved. be able to.

  In the negative electrode member of the present invention, the lithium metal layer formed on these insulating substrates functions as a negative electrode active material, and at the same time functions as a current collector.

  As a result, even if a shortage occurs, the performance of the solid electrolyte membrane deteriorates, and even if local dendrite growth occurs, the supply of electrons automatically stops if the lithium metal in that portion is consumed, This eliminates the risk of repeated intensive charge / discharge. In the present invention, a lithium secondary battery using such a negative electrode member is also provided.

  According to this invention, the base material of the negative electrode member is formed of an electrical insulator or a member provided with an electrical insulator layer on a metal base material, and a lithium metal film and a solid electrolyte film are provided thereon. Dendrite growth caused by the reaction between the metal and the organic electrolyte is suppressed, and even if local dendrite growth occurs, the supply of electrons automatically stops due to the consumption of lithium metal at that site. A short circuit caused by growth is eliminated, and a lithium secondary battery having high energy density, excellent charge / discharge cycle characteristics, and high stability and safety is obtained.

[Example 1]
As shown in FIG. 1, using a copper foil 6 having a thickness of 10 μm and a diameter of 30 mm as a base material, a polypropylene film 7 is cast on the upper surface of the copper foil 6 by leaving a peripheral portion of the upper surface 0.5 mm wide by a casting method. A mask was formed to a thickness of 1 μm.

  Subsequently, a lithium metal film 3 was formed on the entire upper surface by vapor deposition. The thickness of this lithium metal film 3 was 5 μm. The film thickness was measured using a stylus step meter. Furthermore, a solid electrolyte film 4 having a lithium (Li) -phosphorus (P) -sulfur (S) composition was formed on the lithium metal film 3 to a thickness of 0.2 μm by a vapor deposition method to obtain a negative electrode member 5A. As a result of analysis, the solid electrolyte membrane 4 was an amorphous body having a composition of Li 34 atomic%, P 14 atomic%, and S 52 atomic%.

The positive electrode was prepared by mixing LiCoO ₂ particles serving as an active material, carbon particles imparting electron conductivity, and polyvinylidene fluoride together with an organic solvent, and applying the mixture onto an aluminum foil. The active material layer had a thickness of 100 μm, a capacity density of 3 mAh (milliampere · hour) / cm ² , and a total capacity of 21 mAh. The positive electrode had a diameter of 30 mm.

In an argon gas atmosphere with a dew point of −80 ° C. or lower, the negative electrode member 5A, the separator (porous polymer film), and the positive electrode member are placed in a coin-type cell, and an electrolytic salt is added to a mixed solution of ethylene carbonate and propylene carbonate. As an example, 100 lithium secondary batteries were prepared by dropping an organic electrolyte solution in which 1 mol% of LiPF ₆ was dissolved.

  After that, a charge / discharge cycle test was performed on this prototype. The conditions for the cycle test were a 10 mA constant current, a charge of 4.2 V, and a discharge of 3.0 V. The results are shown in Table 1 as Sample No. 1.

  As can be seen from this result, 100 samples No. 1 did not cause an internal short circuit even after 500 cycles. Further, no decrease in capacity was observed, and the yield of non-defective products was 100%.

After the charge / discharge cycle test, the coin cell was disassembled, the negative electrode was taken out, and the negative electrode was observed with a scanning electron microscope (SEM) and subjected to energy dispersive X-ray analysis (EDX).
As a result, it was confirmed that no dendritic growth of lithium metal was observed in the 95 lithium secondary battery negative electrodes, and the solid electrolyte membrane was held on the negative electrode surface.

  Further, with respect to the remaining five battery negative electrodes, it was observed that the solid electrolyte membrane was partially destroyed and dendrite growth occurred locally. Was limited to the vicinity of the surface of the negative electrode.

[Comparative Example 1]
As a comparative test, 100 lithium secondary batteries using a rolled copper foil as a base material and a member having a lithium metal film and a solid electrolyte film formed thereon as a negative electrode were produced and charged and discharged under the same conditions as in Example 1. The cycle test was conducted.

  As a result, 97 batteries stopped at approximately 300 to 500 cycles due to voltage increase. Moreover, about the remaining 3 pieces, the short circuit occurred in about 100 cycles.

  After the charge / discharge cycle test, the coin cell was disassembled and the negative electrode was taken out. The negative electrode was observed with a scanning electron microscope (SEM) and subjected to energy dispersive X-ray analysis (EDX). It was confirmed that no lithium metal dendrite growth was observed for the negative electrode of the battery, and that the solid electrolyte membrane was retained on the negative electrode surface. Was confirmed to reach the positive electrode.

Sectional drawing which shows an example of the negative electrode member of this invention

Explanation of symbols

3 Lithium metal film 4 Solid electrolyte film 5A Negative electrode member 6 Copper foil 7 Polypropylene film

Claims (5)

  In a lithium secondary battery negative electrode member in which a lithium metal film is formed on a base material and an inorganic solid electrolyte film is further formed thereon, the base material is formed of an electrical insulator, and the lithium metal film is formed on the base material. A lithium secondary battery negative electrode member characterized in that a film is formed and the lithium metal film functions as a negative electrode active material and a current collector.   In a lithium secondary battery negative electrode member in which a lithium metal film is formed on a metal substrate and an inorganic solid electrolyte film is further formed thereon, an electrical insulator layer is provided at the interface between the metal substrate and the lithium metal film, A lithium secondary battery negative electrode member, wherein the metal base and the lithium metal film are electrically connected at a peripheral portion of the metal base.   The lithium secondary battery negative electrode member according to claim 1, wherein the electrical insulator is an organic polymer substance.   4. The lithium secondary battery negative electrode member according to claim 3, wherein the organic polymer substance is any one of polyethylene, polypropylene, polyimide, polyamide, polyester, polyether, polyurethane, and polycarbonate.   The lithium secondary battery comprised using the negative electrode member of Claim 1 or 2.

Images