JPH06310124A - Negative electrode for lithium secondary battery - Google Patents

Abstract

PURPOSE:To obtain a negative electrode which can be used to form nemerous forms of batteries handled simply integrally as a sheet form substance and the like, by using a negative electrode active material which consists of a lithium alloy powder while exercising its advantage not growing its dendrite easily, and can form a secondary battery with an excellent battery functions such as the electromotive force property and the charge and discharge cycle life. CONSTITUTION:This negative electrode for lithium secondary battery is composed by binding a negative electrode active matrial 1 which consists of a lithium alloy powder, with a lithium alloy 2 whose melting point is lower than the active material 1, and by fusing and attaching a layer consisting of such a negative electrode to a collector sheet which consists of a heat resistant metal. As a result, a negative electrode with a collector sheet in which the thickness is even, the flatness is excellent, the active material is not removed easily, and the quality is stable, can be obtained. And various forms of lithium secondary batteries which have a large discharge capacity, and an excellent safety or reliability, can be formed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a negative electrode capable of suppressing the growth of dendrites and forming a lithium secondary battery having high electromotive force and excellent battery life.

[0002]

2. Description of the Related Art In a lithium secondary battery using a non-aqueous electrolyte, it is advantageous to form a negative electrode with metallic lithium in terms of energy density and electromotive force. However, since metallic lithium is active, it can be repeatedly charged and discharged. There was a problem that dendrites tended to grow. Dendrites are dendritic crystals in which current is likely to be concentrated. Therefore, once they occur, they grow rapidly and cause a decrease in battery functions such as charge / discharge efficiency and a safety problem due to a short circuit between the positive and negative electrodes.

On the other hand, a negative electrode for a lithium secondary battery using a negative electrode active material composed of a lithium alloy is also known.
This may have lower electromotive force than metallic lithium, but has the advantage that dendrites are less likely to grow.

However, the above-mentioned negative electrode active material made of a lithium alloy has a drawback in that it is inferior in workability, and it is difficult to roll it into a sheet and it is necessary to use it in a powder state, so that it can be pressure-bonded. There is also a problem that there are large restrictions on the application, such as stopping the formation of coin type or button type batteries.

[0005]

DISCLOSURE OF THE INVENTION The present invention makes it possible to handle a dendrite using a negative electrode active material made of a lithium alloy as a sheet-like material in a simple and integrated manner while taking advantage of the advantage that the dendrite does not easily grow. An object is to develop a negative electrode that can be used for forming a battery in any form and can form a lithium secondary battery having excellent battery functions such as electromotive force and charge / discharge cycle life.

[0006]

DISCLOSURE OF THE INVENTION The present invention is characterized in that a negative electrode active material made of a powder of a lithium alloy is bound with a lithium alloy having a melting point lower than that of the negative electrode active material. And a layer composed of such a negative electrode is fusion-bonded to a current collector sheet composed of a heat-resistant metal, and a negative electrode for a lithium secondary battery is provided.

[0007]

[Function] With the above-mentioned structure in which a lithium alloy having a low melting point is bound, a sheet shape or the like that can be easily and efficiently handled as an integrated body without impairing the advantage that the dendrite of the negative electrode active material made of the lithium alloy does not easily grow It is possible to obtain a negative electrode having excellent strength in any of the above. Therefore, it becomes possible to form various types of lithium secondary batteries.

[0008] Further, due to the binding by the lithium alloy, there is a problem of quality variation in the length direction such as when the powder is pressure-pressed and molded into a sheet-like shape, and when the current-collecting sheet is used together, the sheet becomes uneven. Does not occur, and the problem of dropping off due to non-adhesion of the pressure-bonded powder from the sheet at the time of forming the battery does not occur, and the problem of reduction in conductivity and the like due to the surface coating of the powder when binding with an organic binder does not occur. Therefore, according to the above, a negative electrode for a lithium secondary battery having excellent battery functions such as electromotive force and charge / discharge cycle life can be obtained.

[0009]

EXAMPLES A negative electrode used for forming a lithium secondary battery of the present invention is a negative electrode active material made of a powder of a lithium alloy bound with a lithium alloy having a melting point lower than that of the negative electrode. The electrode layer is fusion-bonded to a collector sheet made of a heat-resistant metal.

The negative electrode of the present invention is illustrated in FIGS. 1 and 2. Reference numeral 1 is a negative electrode active material made of lithium alloy powder, 2 is a lithium alloy as a binder, 3 is a negative electrode layer, and 4 is a current collector sheet. The layer composed of the negative electrode may be provided on both sides of the current collector sheet, or may be partially provided on one side or both sides.

The negative electrode active material made of lithium alloy powder is a binary or ternary alloy of Li and a metal such as Al, Pb, Sn, In, Bi, etc., if necessary Si or C
Examples thereof include those to which d, Zn, La and the like have been added, and any known material can be used. It is also possible to use an alloy of Li and Pb in which La or the like is added to improve mechanical properties. What can be preferably used is one having a melting point of 400 ° C. or higher.

Specific examples of the high melting point negative electrode active material include L
i-Al-Si based alloy (for example, Li: Al: Si atomic ratio is 9: 5: 2), Li-Sn-Cd based alloy (for example, Li: Sn: Cd atomic ratio is 9: 2: 1 etc.),
Li-Bi-Cd type alloys (for example, Li: Bi: Cd atomic ratio is 4: 1: 1), Li-Pb-Cd type alloys (for example, Li: Pb: Cd atomic ratio is 4: 1 :). 1 etc.),
An example is a Li-In-Zn-based alloy (for example, Li: In: Zn having an atomic ratio of 3: 1: 1).

The powder made of a lithium alloy as the negative electrode active material may have an appropriate particle size depending on the form of the electrode to be formed. Generally, less than 30 μm, especially 0.1
An average particle size of about 10 μm is used. If the average particle size exceeds 30 μm, it may be difficult to form a homogeneous electrode.

The lithium alloy used for binding the negative electrode active material may have a melting point lower than that of the lithium alloy as the negative electrode active material. Preferably, the melting point is 300 ° C
Hereinafter, a lithium alloy having a temperature of 200 ° C. or less is used.

An example of a lithium alloy having a melting point of 300 ° C. or lower is Li and, for example, Ag, Ba, Ca, Hg, Pd, P.
Examples include alloys of binary or ternary or more with metals such as t and Sr. Specific examples thereof, Li-Ag _{alloy, Li 6 Hg, Li 4 Ba} , Li 2 Ca, Li 5 Pd, Li 5 P
t, Li ₂₃ Sr _{6 and the} like.

The proportion of the lithium alloy used for binding is 1 to 30% by weight of the lithium alloy powder for the negative electrode active material, especially 5
-15% by weight is preferred. If the usage ratio is less than 1% by weight, the binding strength may be weak and the mechanical strength may be poor.
If it exceeds 0% by weight, the negative electrode active material may not function easily and the electrode characteristics may be poor.

For forming the negative electrode of the present invention, for example, a powder of a lithium alloy as a negative electrode active material and a powder of a lithium alloy for binding are mixed through a dispersion medium such as alcohols or glycols which is easy to dry. , The mixed dispersion is molded by an appropriate method such as a casting method or a coating method to volatilize the dispersion medium and then heated to melt the lithium alloy for binding,
Then, it can be carried out by a method of cooling and solidifying.

In the case of the type in which the negative electrode layer is closely attached to the current collecting sheet, for example, the above-mentioned mixed dispersion liquid is spread on the current collecting sheet by an appropriate method such as a coating method to volatilize the dispersion medium. Heat to melt the lithium alloy for binding,
Then, it can be carried out by a method of cooling and solidifying. The fusion bonding of the negative electrode layer with the current collecting sheet is satisfactorily achieved in the cooling and solidifying process.

As the current collector sheet, one made of an appropriate heat-resistant metal (including alloy) that can withstand the heating temperature for melting the above-mentioned lithium alloy for binding may be used. Generally, a material having excellent conductivity such as aluminum or copper is used. The thickness of the collector sheet is appropriately determined according to the purpose of use of the electrode, etc., and is generally 0.05 to 0.5 mm,
Especially, it is 0.1 to 0.3 mm. The thickness of the negative electrode layer provided on the current collector sheet is also arbitrary and may be appropriately determined depending on the purpose of use of the electrode, etc., and is generally 0.05 to 0.4 m.
m, especially 0.05 to 0.2 mm.

The lithium alloy powder for binding used in the preparation of the above-mentioned mixed dispersion has a smaller average particle size than the powder of the lithium alloy as the negative electrode active material, but is advantageous in forming a homogeneous electrode. More preferably used. Generally, 1/10 to 1 / l of the lithium alloy powder for the negative electrode active material
Those having an average particle size of about twice are used for binding.

In any of the above cases, if the average particle size of the lithium alloy powder is less than 0.01 μm, the lithium alloy powder easily scatters, resulting in poor workability such as handling.
It is preferable to use a powder having a size of μm or more. The lithium alloy powder can be formed by an appropriate method such as a crushing method using a ball mill or a gas atomizing method.

The electrode of the present invention is for forming a negative electrode of a lithium secondary battery, but there is no particular limitation on the formation of the lithium secondary battery except that such a negative electrode is used, and an organic solvent is used. It can be carried out according to the conventional method using the non-aqueous electrolyte and the positive electrode.

Therefore, as a typical example of such a non-aqueous electrolyte, a lithium salt is dissolved in an appropriate organic solvent such as dimethoxyethane, tetrahydrofuran, acetonitrile, propylene carbonate and, if necessary, 2-methylfuran,
Examples thereof include those in which organic additives such as thiophene, pyrrole and crown ether are dissolved. Examples of the lithium salt include LiI, LiCF ₃ SO ₃ , LiBF ₄ , L
Examples thereof include iClO ₄ , LiAlCl ₄ , LiPF ₆ , and LiAsF ₆ .

Further, as the positive electrode, an appropriate one such as carbon or metal can be used. A positive electrode that can be preferably used from the viewpoint of high electromotive force has a general formula: Li-Co-PO composite oxide as an active material.

Charging of the obtained lithium secondary battery can be carried out not only by a method of continuously supplying a constant current, but also by a method of supplying a pulse current using an appropriate pulse power source. The charging method using a pulse current has the advantage that dendrites are more difficult to grow because the change in concentration of the non-aqueous electrolyte is suppressed because energization / stopping is repeated.

Example 1 Powder having an average particle size of 15 μm and made of a Li—Al—Si type alloy having an atomic ratio of Li: Al: Si of 9: 5: 2 (negative electrode active material).
And 7% by weight of Li ₅ Pd (binder) having an average particle size of 5 μm were mixed in a slurry form using a mixed dispersion medium of ethylene glycol and isopropyl alcohol, and a powder having a thickness of 200 μm was mixed. It is applied on an aluminum sheet, the dispersion medium is volatilized under heating, then heated to 190 ° C. to melt the Li ₅ Pd powder, and then cooled and solidified to form Li.
-Al-Si alloy powder is bound by Li ₅ Pd layer to a thickness of 1
An electrode having a negative electrode layer of 10 μm was obtained. The negative electrode layer was well fused on the aluminum sheet in a state where it could not be peeled off by hand.

Example 2 The same as Example 1 except that a Li—Sn—Cd based alloy having an Li: Sn: Cd atomic ratio of 9: 2: 1 was used in place of the Li—Al—Si based alloy powder. Thus, a negative electrode in which the negative electrode layer and the aluminum sheet were well fused was obtained.

Comparative Example 1 A negative electrode was obtained in the same manner as in Example 1 except that polyvinylidene fluoride was used as the binder instead of the Li ₅ Pd powder.

Comparative Example 2 A negative electrode was obtained in the same manner as in Example 2 except that polyvinylidene fluoride was used as the binder instead of the Li ₅ Pd powder.

Evaluation test On top of the negative electrode obtained in the examples or comparative examples, 1 of LiBF ₄
A lithium secondary battery was formed by disposing a positive electrode made of LiCo _0.75 P _0.25 O ₂ powder through a separator obtained by impregnating a polypropylene non-woven fabric with a propylene carbonate solution having a mol / liter concentration.
Charging / discharging at a voltage of 4.2 V between electrodes and discharging to an electromotive force of 2.0 V at mA / cm ² was repeated 100 cycles, and the discharge capacity and charging / discharging efficiency at the end of the cycle were examined.

The above results are shown in Table 1.

[Table 1]

[0032]

According to the present invention, it is possible to obtain a negative electrode in which a dendrite is hard to grow and which is excellent in strength and homogeneity in any form such as a sheet which can be easily and efficiently handled as an integrated body. it can. Further, it is possible to obtain a negative electrode with a current collector sheet which is excellent in thickness uniformity and flatness, and in which quality of the active material is less likely to drop off and which is stable. As a result, it is possible to form various types of lithium secondary batteries having a large discharge capacity and excellent battery functions such as electromotive force and charge / discharge cycle life, and excellent safety and reliability.

[Brief description of drawings]

FIG. 1 is a sectional view of an example.

FIG. 2 is a sectional view of another embodiment.

[Explanation of symbols]

 1: Negative electrode active material made of lithium alloy powder 2: Lithium alloy as a binder 3: Negative electrode layer 4: Current collecting sheet

Claims (2)

[Claims] 1. A negative electrode for a lithium secondary battery, comprising a negative electrode active material made of lithium alloy powder and a lithium alloy having a melting point lower than that of the negative electrode active material. 2. A negative electrode for a lithium secondary battery, wherein a layer made of the negative electrode for a lithium secondary battery according to claim 1 is fusion-bonded to a current collector sheet made of a heat-resistant metal. .