JPH0722017A - Alloy for lithium secondary battery negative electrode and lithium secondary battery provided with negative electrode made of this alloy - Google Patents

Abstract

PURPOSE:To provide a lithium secondary battery excellent in cycle lifetime of electric charging and discharging, and realizing high electromovitve force, a high electric discharging capacity, and high energy density by using an Li-X- Te alloy having a specific composition (wherein X represents one kind of metal or an alloy composed of metals selected from Ag, Zn, Ca, Al, and Mg). CONSTITUTION:A negative electrode 1 is made of an Li-X-Te alloy composed in a composing ratio of Li:X:Te=80-150:1-20:2-30, wherein X represents metal or an alloy selected from Ag, Zn, Ca, Al and Mg. The alloy is rolled into an alloy sheet, which is bonded to a nickel sheet, thus obtaining the negative electrode 1. A separator 3 impregnated with an electrolyte is interposed between the negative electrode 1 and a positive electrode 2. A stainless steel cap 5 to be brought into press-contact with a current collector 4a press-fitted to the outside surface of the negative electrode 1 and a stainless steel positive electrode can 6 to be brought into press-contact with another current collector 4b press- fitted to the outside surface of the positive electrode 2 are sealed with a gasket 7, thereby obtaining a lithium secondary battery D.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery negative electrode material, a lithium secondary battery negative electrode alloy, and a lithium secondary battery provided with a negative electrode formed from the negative electrode alloy.

[0002]

2. Description of the Related Art Generally, secondary batteries are required to have high energy density, high output density, and low self-discharge rate.
It is inexpensive, has high energy efficiency, and has a long cycle life. As a secondary battery having such performance, a non-aqueous electrolyte battery utilizing electric energy due to movement of lithium ions, a so-called lithium secondary battery is known to have a high electromotive voltage and a high energy density.

In this lithium secondary battery, when pure lithium is used for the negative electrode, a negative electrode having a high energy density can be obtained, but on the other hand, dendrites are likely to grow during charging. Electrodeposition of metal generally proceeds in two stages of mass transfer (diffusion) of metal ions to the electrode surface and exchange of electrons at the electrode (electrode reaction), but in the electrodeposition of pure lithium, the electrode reaction is As soon as possible, electrodeposition is limited by the diffusion of lithium ions to the electrode surface. In electrodeposition controlled by such mass transfer, dendrites generally grow easily. This dendrite is a dendritic crystal, and once formed, it grows rapidly and penetrates the separator to short-circuit with the positive electrode, resulting in a significantly shortened cycle life of the battery.

In order to solve this problem, it has been conventionally used to use a lithium alloy for the negative electrode. The components of this lithium alloy include Al, Bi, Pb, Sn and In.
Although metal elements such as are known, alloys using these metal elements are all intermetallic compounds, and the electrode formation rate is controlled by diffusion of lithium by reducing the electrode reaction rate in lithium electrodeposition by alloy formation. This is alleviated and the growth of dendrites is suppressed.

However, since the intermetallic compound as described above is a brittle material, cracks occur in the electrode due to expansion / contraction of the electrode volume due to absorption / desorption of lithium during charging / discharging, and finally powder. Leading to Further, since the negative electrode made of an intermetallic compound contains a large amount of metal elements and has a higher electrode potential than the negative electrode made of pure lithium, there is a drawback that the battery electromotive force is low.

On the other hand, as an alloy negative electrode having a high electromotive force, a negative electrode made of a solid solution alloy prepared by adding a small amount of Ag, Zn, etc. to lithium has been proposed. Under the present circumstances, it cannot be said that the battery using this has sufficient charge / discharge characteristics because of the problems such as the loss of components.

An object of the present invention is to solve the above problems and to provide an alloy for a negative electrode of a lithium secondary battery, which has a large charge / discharge capacity, a high energy density and little deterioration due to repeated charge / discharge. It is in. Another object of the present invention is to provide a lithium secondary battery having high electromotive force, high charge / discharge capacity, high energy density, and excellent cycle life.

[0008]

As a result of extensive studies on alloys for lithium negative electrodes, the present inventor has found that the above object can be achieved when the negative electrode is made of an alloy having a specific composition. It came to completion. That is, the alloy for a lithium secondary battery negative electrode of the present invention is a Li-X-Te based alloy (where X is one metal selected from Ag, Zn, Ca, Al and Mg or two or more alloy components). And the Li-X-Te based alloy composition has an atomic ratio of Li: X: Te = 80 to 150: 1 to 20: 2 to 30.
It is characterized by being. Further, the lithium secondary battery of the present invention is characterized by including a negative electrode made of the above alloy for a negative electrode of the lithium secondary battery.

One kind of metal selected from Ag, Zn, Ca, Al and Mg used in the alloy for the negative electrode of the lithium secondary battery of the present invention or two or more kinds of alloy components (hereinafter referred to as metal or alloy component X). Has a function of reacting with lithium to form a lithium alloy and absorbing and releasing lithium during charge and discharge.

On the other hand, Te is a stable intermetallic compound X _a Te _b (Ag ₂ Te, ZnT) with the above metal or alloy component X.
e, CaTe, Al ₂ Te ₃ , MgTe, etc.),
It also forms an intermetallic compound Li ₂ Te with lithium. These intermetallic compounds have the effect of significantly refining the crystal grains of the negative electrode alloy. This action significantly increases the area of the crystal grain boundary, but the lithium diffusion rate at the crystal grain boundary reaches several tens of times or more that in the grain. Therefore, the fact that the lithium electrodeposition is rate-limited by the diffusion of lithium is relaxed, and the growth of dendrites is suppressed. Further, this allows lithium to be efficiently absorbed and released. In addition, these intermetallic compounds form a three-dimensional shell by accumulating at grain boundaries. The above intermetallic compound X _a Te _b
Of Li ₂ Te and XaTe _b , X _a Te _b functions as an aggregate that retains the strength of the shell or a binder of Li ₂ Te. On the other hand, the diffusion rate of lithium in Li ₂ Te is extremely high,
Li ₂ Te functions as a high-speed diffusion path for lithium, and the absorption and desorption of Li accompanying the charging and discharging of the battery proceeds through the shell. Further, the shell also functions as a diffusion barrier for the metal or alloy component X, which prevents the metal or alloy component X from falling off the electrode during discharge. Furthermore, the electrode potential of Li ₂ Te is equivalent to that of lithium, and therefore the battery electromotive force is equivalent to that using pure lithium.

The alloy for a negative electrode of a lithium secondary battery of the present invention comprises
Lithium: metal or alloy component X: Te = 80 in atomic ratio
˜150: 1 to 20: 2 to 30 are reacted to form an alloy. If the metal or alloy component X is less than 1 in the above composition, the grain refining effect becomes insufficient, and the strength of the shell formed by the intermetallic compound decreases, which is not preferable. On the other hand, the above metal or alloy component X
When it exceeds 20, it becomes difficult to process it into a sheet shape, and the electrode potential rises to lower the battery electromotive force, which is not preferable. Further, in the above composition, when Te is less than 2, dendrite is not sufficiently suppressed, and the grain refinement effect is insufficient, which is not preferable. On the other hand Te
When it exceeds 30, it becomes difficult to process it into a sheet shape, and the electrode potential rises to lower the battery electromotive force, which is not preferable.

The alloy for the negative electrode of the lithium secondary battery is Li
It is obtained by alloying a metal with a metal or alloy component X and Te. This alloying is performed by a conventional method such as a method of melting and reacting these alloy materials or a method of reacting them by physical vapor deposition. This is done by a known method.

The alloying by the melting method is performed by heating and melting the above alloy material in an inert atmosphere. In the above heating and melting, it is preferable to carry out the heating at a temperature equal to or higher than the melting point of Li, because the reaction proceeds rapidly.

Alloying by vapor deposition evaporates the metal,
This is done by solidifying on other metal surfaces. According to this vapor deposition method, a so-called non-equilibrium phase alloy of a high melting point metal and a low melting point metal that cannot be alloyed by the melting method can be produced. As the vapor deposition method, various kinds of sputtering such as ion beam sputtering, electron beam vapor deposition,
There are various ion plating methods, CVD methods, etc., but in the case of producing the alloy of the non-equilibrium phase as described above, it is carried out by a method of accelerating atoms or ions toward the substrate, such as ion plating and sputtering. It is preferable.

The lithium secondary battery of the present invention is characterized by using the lithium alloy described above as a negative electrode material. The negative electrode forming the lithium secondary battery is obtained by molding the lithium alloy into a sheet shape, a thin film shape, or the like by a usual method. When the above negative electrode material is formed into a sheet, it is usually formed into a thickness of several μm to several hundreds of μm by a method such as rolling or hot extrusion.

The sheet-shaped negative electrode is used by being joined to a sheet-shaped current collector such as a Ni sheet, an Al sheet or a Cu sheet by various joining methods such as brazing, soldering, ultrasonic welding or spot welding. .

On the other hand, as the positive electrode material constituting the positive electrode of the lithium secondary battery of the present invention, a positive electrode material usually used for the positive electrode of a lithium secondary battery can be used. For example, MnO ₂ or LiCo ₂ O 3
A positive electrode material having ₂ or the like as an active material can be preferably used. In addition,
The positive electrode active material is mixed with a conductive material such as acetylene black or Ketjen black, and a binder such as polytetrafluoroethylene or polyethylene.

The positive electrode mixture obtained by mixing the positive electrode active material with a conductive material and a binder is molded into an appropriate shape and size by any method such as casting, compression molding, roll molding, It is used as the positive electrode of the lithium secondary battery of the present invention.

As the electrolyte used in the lithium secondary battery of the present invention, an electrolytic solution in which salts are dissolved in an organic solvent or a solid electrolyte can be used. When the electrolyte is an electrolytic solution, the salts include LiClO ₄ , LiBF ₄ , LiPF ₆ , and L.
iAsF ₆ , LiAlCl ₄ , Li (CF ₃ SO ₂ ) ₂
N, etc. can be used, ethylene carbonate, propylene carbonate, dimethyl sulfoxide, sulfolane, γ-
It is dissolved in an organic solvent such as butyrolactone, 1,2-dimethoxyethane, N, N-dimethylformamide, tetrahydrofuran, 1,3-dioxolane, 2-methyltetrahydrofuran, diethyl ether and a mixture thereof to give a concentration of 0.
It is used by adjusting to 1 to 3 mol / liter. This electrolytic solution is usually used by impregnating or filling a separator such as a porous polymer or a glass filter.

When the electrolyte is a solid electrolyte, the above-mentioned salts are mixed with polyethylene oxide, polyphosphazene, polyaziridine, polyethylene sulfide and the like, their derivatives, mixtures and complexes. This solid electrolyte also serves as a separator for the negative electrode and the positive electrode.

[0021]

The alloy for the negative electrode of the lithium secondary battery of the present invention is Li-
It is a solid solution type alloy composed of an X-Te alloy, and the metal or alloy component X has a function of reacting with lithium to form a lithium alloy and absorbing / releasing lithium during charge / discharge. This promotes the diffusion of lithium during charge / discharge. Meanwhile, Te forms the metal or alloy component X and stable intermetallic compound X _a Te _b, also forms an intermetallic compound with lithium Li ₂ Te. These intermetallic compounds significantly reduce the size of the crystal grains of the negative electrode alloy, which significantly increases the area of the crystal grain boundaries, and the lithium diffusion rate at the crystal grain boundaries reaches several tens of times or more that in the grains. . This alleviates that the lithium electrodeposition is rate-limited by the diffusion of lithium. Further, this allows lithium to be efficiently absorbed and released.
In addition, these intermetallic compounds form a three-dimensional shell by accumulating at grain boundaries. Of these, X _a Te _b functions as an aggregate that retains the strength of the shell or a binder of Li ₂ Te.
Whereas Li ₂ diffusion rate of lithium in the Te significantly faster, Li ₂ Te functions as a high speed diffusion path of lithium. For this reason, the absorption of lithium as the battery is charged and discharged
The release proceeds through the shell, and the shell is composed of the intermetallic compound, so that dendrite growth is suppressed. The shell also functions as a diffusion barrier for the metal or alloy component X. Further, the electrode potential of Li ₂ Te is equivalent to that of lithium.

[0022]

EXAMPLES Examples of the present invention will now be described in more detail. Needless to say, the present invention is not limited to this. Example 1 (Production of Negative Electrode) Li: Ag and Te in atomic ratio Li: A
It was weighed so that g: Te = 105: 7: 6, and heated and melted at 500 ° C. in an argon atmosphere to form an alloy. This alloy was cooled and rolled in a dry air atmosphere to obtain an alloy sheet having a thickness of 1 mm. This alloy sheet has a diameter of 20.
A disk-shaped negative electrode was prepared by punching into a 0 mm circular shape and bonding it to a circular nickel sheet having a thickness of 0.5 mm.

(Production of lithium secondary battery) Li: Co: P = 2: 1.5: 0.5 in atomic ratio of lithium carbonate, basic cobalt carbonate, and phosphoric acid aqueous solution having a phosphoric acid content of 85%.
Each of the above amounts was weighed and thoroughly mixed, then placed in an alumina crucible and heat-treated at 900 ° C. for 24 hours to produce an oxide substance. The oxidant is a mixture of lithium phosphate, lithium cobalt phosphate and cobalt oxide. A positive electrode mixture was prepared by sufficiently mixing 8 parts by weight of the positive electrode active material obtained by pulverizing the above mixture and adjusting the particle size to 20 μm or less, 1 part by weight of acetylene black, and 1 part by weight of Teflon powder. Then, this positive electrode mixture is press-molded on a nickel mesh to have a diameter of 2
A disk-shaped positive electrode having a thickness of 0 mm and a thickness of 1.0 mm was produced. Separately, an electrolyte solution was prepared by dissolving 1 mol / liter of lithium perchlorate in propylene carbonate having a water content adjusted to 50 ppm or less. Further, a porous polypropylene film having a thickness of 0.5 mm was punched into a diameter of 25.0 mm to prepare a separator.

Using the negative electrode, the positive electrode and the separator, as shown in the schematic view of FIG. 1, a separator 3 impregnated with an electrolytic solution is interposed between the negative electrode 1 and the positive electrode 2 to form an outer surface of the negative electrode 1. A negative electrode cap 5 made of stainless steel, which comes into pressure contact with the current collector 4a pressure-bonded to it, and a positive electrode can 6 made of stainless steel which comes into pressure contact with the current collector 4b pressure-bonded to the outer surface of the positive electrode 2, are sealed with a gasket 7, and lithium A secondary battery D was produced.
When the electromotive force of the lithium secondary battery was measured by the two-terminal method, it was as shown in Table 1.

(Charge / Discharge Test) Using the above-mentioned test lithium secondary battery, the upper limit voltage of 4.2 at a current density of 1 mA / cm ^2.
V and the lower limit voltage of 2 V were set, and charging / discharging was repeated 50 times. When the discharge capacity and charge / discharge efficiency at the 50th charge / discharge cycle were examined, the results were as shown in Table 1.

Comparative Example 1-1 A negative electrode was prepared in the same manner as in Example 1 except that Te was omitted.

Comparative Example 1-2 A negative electrode was prepared in the same manner as in Example 1 except that Te was omitted and the Li alloy composition was Li: Ag = 95: 5.

Example 2 Li: Zn: Te = 10 in atomic ratio of Li: Zn: Te
Weigh it so that it is 9: 2: 6, and 5 in an argon atmosphere.
It was heated and melted at 00 ° C to form an alloy. This alloy was cooled and rolled in a dry air atmosphere to obtain an alloy sheet having a thickness of 1 mm. This alloy sheet was punched into a circular shape having a diameter of 20.0 mm and bonded to a circular nickel sheet having a thickness of 0.5 mm to produce a disk-shaped negative electrode.

Comparative Example 2-1 A negative electrode was prepared in the same manner as in Example 2 except that Te was omitted.

Comparative Example 2-2 A negative electrode was prepared in the same manner as in Example 2 except that Te was omitted and the Li alloy composition was Li: Zn = 98: 2.

Example 3 Li, Ag and Te were atomic ratio Li: Ag: Te = 105: 7: 6 on a nickel substrate having a thickness of 0.5 mm by ternary vapor deposition by the cluster ion beam method. Then, an alloy film having a thickness of 10 μm was formed by vapor deposition. Here, the substrate temperature was 150 ° C. The substrate on which the alloy film was formed was punched out to a diameter of 20.0 mm to produce a negative electrode.

Comparative Example 3-1 A negative electrode was prepared in the same manner as in Example 3 except that Te was omitted.

Comparative Example 3-2 A negative electrode was prepared in the same manner as in Example 3 except that Te was omitted and the Li alloy composition was Li: Ag = 95: 5.

Examples 4 to 7 In the above Example 1, the alloy composition and composition ratio are shown in Table 1.
A negative electrode was prepared in the same manner except that the negative electrode was changed as shown in FIG.

(Preparation of Lithium Secondary Battery and Charge / Discharge Test) Test lithium batteries were prepared in the same manner as in Example 1 except that the negative electrode bodies obtained in Examples 2 to 7 and Comparative Examples 1 to 3 were used. The following batteries were produced, and the electromotive force of each lithium secondary battery was measured by the two-terminal method. In addition, using each of the above lithium secondary batteries,
Charge and discharge were performed in the same manner as in Example 1, and the discharge capacity and the charge and discharge efficiency at the 50th charge and discharge were examined, and the results are shown in Table 1.

[0036]

[Table 1]

[0037]

As described above in detail, the lithium of the present invention
The secondary battery negative electrode alloy is composed of a Li-X-Te-based alloy.
It is a solid solution type alloy, and contains metal or alloy component X and lithium.
The lithium alloy formed from the
Since it has the function of absorbing and releasing thium,
Inhibition of dendrite growth by promoting diffusion
Has become. On the other hand, Te and the above metal or alloy component X
An intermetallic compound X formed from _aTe_bAnd
An intermetallic compound Li formed from Te and lithium
₂Due to the crystal grain refining action of Te, the crystal grains of the negative electrode alloy
The area of the boundary increases significantly, and the lithium diffusion rate at the grain boundaries
The speed is several tens of times faster than in a grain. this
As a result, the negative electrode alloy has a lithium electrodeposition due to the diffusion of lithium.
The rate-limiting is alleviated, and lithium is efficient
It is well absorbed and released. Therefore
The growth of dendrites is also suppressed by the action of
It In addition, these intermetallic compounds accumulate at the grain boundaries and stand up.
Of the above intermetallic compounds that make up the physical shell,
X_aTe_bBecomes an aggregate and maintains the mechanical strength of the shell,
Li₂Since it functions as a binder for Te,
The strength of gold has improved. On the other hand, Li₂Te is lithium
As it functions as a high-speed diffusion path, it is
Lithium is efficiently absorbed and released, which
Battery discharge capacity has increased and charging time has also been shortened.
It In addition, the absorption and release of lithium progress through the shell.
And the shell is composed of the above intermetallic compound
Therefore, the growth of dendrites is suppressed. Well
Further, the shell serves as a diffusion barrier for the metal or alloy component X.
Also works, which allows the metal or alloy component X to
It is prevented from falling off from the electrode, and it is possible to repeat charging and discharging.
Further, the negative electrode is prevented from being deteriorated. Furthermore, L
i₂Since the electrode potential of Te is the same as that of lithium,
The electromotive force is equivalent to that using pure lithium.
Therefore, from the alloy for a lithium secondary battery negative electrode of the present invention
A lithium secondary battery with a negative electrode
Long life, high electromotive force, high discharge capacity and high energy
-It has a density.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view of a lithium secondary battery showing an embodiment of the present invention.

[Explanation of symbols]

 1 Negative electrode 2 Positive electrode 3 Separator D Lithium secondary battery

Claims (2)

[Claims] 1. A Li-X-Te-based alloy (where X is A
An alloy for a lithium secondary battery negative electrode comprising one metal selected from g, Zn, Ca, Al, and Mg or two or more alloy components), wherein the Li-X-Te-based alloy composition has an atomic ratio of Li: X: Te = 80 to 150: 1 to 20: 2
The alloy for a negative electrode of a lithium secondary battery, which is 30. 2. A lithium secondary battery comprising a negative electrode made of the lithium alloy according to claim 1.