JPH07326342A - Negative electrode for lithium secondary battery, and lithium secondary battery using the same - Google Patents

Abstract

PURPOSE:To provide a negative electrode for lithium secondary battery which has a large charge/discharge capacity and a high electromotive force, and is minimized in the deterioration by repeat of charge/discharge, and a lithium secondary battery using this negative electrode. CONSTITUTION:This negative electrode A for lithium secondary battery has a laminated body having a porous layer 3 consisting of Li alloy formed on the surface of a carbon layer 2. The Li alloy used in the porous layer 3 has a characteristic that the speed of absorption and release of Li is slow, compared with the carbon used in the carbon layer 2, and Li-Si alloy or Li-Zn alloy is particularly preferred. The composition ratio of each alloy is most preferably about Li:Si=1:(3.3-4.4) in the Li-Si alloy and about Li:Zn=l:(1-1.5) in the Li-Zn alloy.

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, and more specifically, it has high electromotive force and large discharge capacity, and
The present invention relates to a negative electrode for a lithium secondary battery, which has an excellent cycle life related to charging and discharging, and a lithium secondary battery using the negative electrode.

[0002]

2. Description of the Related Art Li is an excellent material as an active material for a negative electrode of a secondary battery in terms of charge / discharge capacity and electromotive force. However, lithium has a property of easily growing as acicular crystals (dendrites) during deposition on the negative electrode during charging, and the dendrites cause various problems.
When the above Li dendrite grows and repeats growth and lack, the lack of Li is often unrelated to charge and discharge because the supply of electric charge is cut off. Therefore, Li is consumed more than necessary, and the charge / discharge cycle characteristics deteriorate. Further, when the Li dendrite grows continuously without being lost, the needle-like crystals finally come into contact with the positive electrode and cause a short circuit trouble. Conventionally, in order to solve these problems, it has been made to use carbon or a Li alloy (Li-Si, Li-Zn, etc.) as a material for the negative electrode. However, the carbon negative electrode has a problem of low electromotive force, although a relatively large discharge capacity can be obtained.
On the other hand, a negative electrode made of a Li alloy such as Li-Si or Li-Zn exhibits a high electrode potential comparable to that of Li and generates a high electromotive force, but since the Li absorption / desorption rate is low, the charge / discharge capacity is low. There is a problem that is small and not practical.

[0003]

SUMMARY OF THE INVENTION An object of the present invention is to provide a negative electrode for a lithium secondary battery, which has a large charge / discharge capacity and a high electromotive force and which is less deteriorated due to repeated charge / discharge. Another object of the present invention is to provide a lithium secondary battery having a large capacity, high electromotive force, and excellent charge / discharge cycle life using the above-mentioned negative electrode for a lithium secondary battery.

[0004]

The negative electrode for a lithium secondary battery of the present invention has the following features. (1) A negative electrode for a lithium secondary battery, which has a laminated body in which a porous layer made of a Li alloy is formed on a carbon layer as an active material layer. (2) The negative electrode for a lithium secondary battery according to (1), wherein the Li alloy is a Li-Si alloy or a Li-Zn alloy. (3) The composition ratio of the Li-Si alloy (the ratio of the number of component elements, the same applies below) is Li: Si = 1: (3.3 to 4.4), and the composition ratio of the Li-Zn alloy is Li: Zn = 1: (1
~ 1.5) (2) The negative electrode for a lithium secondary battery according to (2).
A lithium secondary battery of the present invention is characterized by including the negative electrode for a lithium secondary battery according to any one of (1) to (3) above.

A structure for enabling the negative electrode for a lithium secondary battery of the present invention to serve as a negative electrode having both characteristics of carbon and Li alloy will be described below with reference to the drawings. Figure 1
It is a figure which partially expands and shows typically an example of the negative electrode for lithium secondary batteries (henceforth "a negative electrode") of this invention as some lithium secondary batteries. As shown in the figure, the constitution of the negative electrode A of the present invention is characterized in that it has, as an active material layer 1, a laminate in which a porous layer 3 made of a Li alloy is formed on a carbon layer 2. It is a thing. Further, in the figure, 4 is a large number of holes present in the porous layer 3 made of a Li alloy, 5 is a conductor layer, 6 is an electrolytic solution, and the surface of the carbon layer 2 becomes an electrolytic solution 6 by the holes 4. Exposed. Further, the lithium secondary battery of the present invention uses the negative electrode for a lithium secondary battery of the present invention, and combines the negative electrode with predetermined essential elements such as a positive electrode, a separator, an electrolyte, a case, etc. Is a secondary battery formed as.

[0006]

By configuring the negative electrode as described above, carbon and Li alloy are integrated in the same electrolytic solution and electrically connected to be present as one negative electrode, and as shown in FIG. In addition, when viewed from the side of the electrolytic solution 6, the carbon layer 2 exists at the innermost part of the pores 4 of the porous layer 3. With such a configuration, the following three main actions are shown. (1) In the charging / discharging process, Li ions in the electrolytic solution do not deposit on the Li alloy of the porous layer, but pass through the pores in the Li alloy and are preferentially deposited on the carbon layer. . This action is a feature in the material selection aspect of the present invention, and is because the Li alloy has a characteristic that the absorption and desorption rates of Li with respect to the electrolyte are slower than that of carbon. Specifically, this is because the polarization resistance of anodic dissolution and electrodeposition of Li on the surface of the Li alloy is larger than that on the surface of carbon. When a Li alloy and carbon are used integrally as the negative electrode active material layer, Li alloy is less likely to absorb and release Li as compared with carbon. Therefore, Li electrodeposition and anodic dissolution in the negative electrode active material layer are It proceeds preferentially on the carbon surface. (2) The carbon layer is responsible for a large discharge capacity, and the porous layer made of Li alloy is responsible for generating a high electromotive force. This action is obtained by the feature of the structural aspect of the present invention, and while utilizing the action of preferentially depositing Li on the carbon surface, which is the feature of the material selection aspect of (1) above, This is an action contrary to this, that is, the property of carbon and the property of the above Li alloy are compatible with each other. That is, in order for Li ions to reach the carbon layer, they must always pass through the pores of the Li alloy, so that the electrochemical reaction of Li ions is reversible even at the interface between the Li alloy and the electrolytic solution. The carbon layer and the Li alloy layer share the negative electrode function. That is, Li preferentially precipitates with respect to carbon, but the existing value of the Li alloy is not lost even by this, and the characteristics can be exhibited. (3) Since the surface area of the entire active material layer is sufficiently large, the current density on the surface is small. This action is
It is not simply obtained by the porosity of the active material layer, but is established by the actions of (1) and (2) above. Thereby, Li is preferably dispersed and concentrated precipitation is suppressed.

As the carbon material used for the carbon layer, those having high crystallinity are preferable, and graphite, natural graphite and the like are exemplified.

As the Li alloy forming the porous layer,
There is no particular limitation as long as the object of the present invention can be achieved, and examples thereof include the following. Li-M1-T
e-based alloy. However, M1 is Ag, Zn, Ca, Al, M
One metal or two or more alloy components selected from g, etc., and their composition ratio (ratio of the number of component elements, the same applies hereinafter)
, Li: M1: Te = 80 to 150: 1 to 20: 0.
001 to 2. Li-M2-Cd type alloy. However, M
2 is one kind of metal or two or more kinds of alloy components selected from Sn, Bi, Pb, In, etc., and the composition ratio thereof is
Li: M2: Cd = 70-90: 10-30: 30-10-3
It is 0. Li-M3-Si based alloy. However, M3 is A
One kind of metal or two or more kinds of alloy components selected from rare earth metals such as l, Y, Fe and Er, and the composition ratio thereof is Li: M3: Si = 5 to 7: 0.5 to 2: 1. ~ 2. Li-In-Zn alloy (composition ratio Li: In: Z
n = 2 to 6: 0.5 to 1.5: 0.5 to 1.5). Li
-Ag-based alloy (composition ratio Li: Ag = 80 to 99: 1 to 2)
0) Others, Li ₆ Hg, Li ₄ Ba, Li ₂ Ca, Li ₅
Examples include Pd, Li ₅ Pt, Li ₂₃ Sr _{6 and the} like. Moreover, Li-Si and Li-Zn are preferable negative electrode materials with which high electromotive force can be obtained. The composition ratio of the Li-Si alloy is
Li: Si = 1: (1 to 10) or so, preferably Li:
Si = 1: (3-5), especially Li: Si = 1: (3.3
.About.4.4) is preferable in that a high electromotive force is maintained while suppressing the precipitation of metallic lithium crystals. Li-
The composition ratio of the Zn alloy is Li: Zn = 1: (0.5 to 5).
The degree, preferably Li: Zn = 1: (0.8 to 3), especially Li: Zn = 1: (1 to 1.5), is the above Li—Si.
It is preferable for the same reason as for the alloy.

The purpose of making the layer made of the Li alloy porous is to expose the surface of the carbon layer to the electrolytic solution 6 at the innermost of the holes inside the Li alloy as described above. Therefore, the term "porosity" as used herein means that a large number of fine pores are irregularly or regularly present in the material, and the electrolytic solution penetrates into the pores from one surface of the material, It is preferable to have good communication with the surface (contact surface with the carbon layer). The porosity of the Li alloy layer having a porosity of about 20 to 80%, preferably 30 to 70%, and particularly preferably 50 to 60% in porosity does not lower the electromotive force. It is suitable for the reason that it enhances the ion diffusivity.

The method for forming a porous layer using the above Li alloy is not particularly limited, but various vapor deposition methods are used, and particularly preferable methods are a sintering method, a pressure casting method, A thermal spraying method can be mentioned.

The sintering method is a method of forming a porous material by heating and diffusion-bonding raw metal particles. In this method, first, by the gastomize method, L
Particles of i-alloy are prepared. The gastomize method is a method in which molten metal is injected into a reduced pressure atmosphere through a nozzle to produce metal particles. A volatile organic solvent is added to a certain amount of the particles of the Li alloy to impart fluidity to form a paste, and the paste is spread on a conductor layer tape made of a good conductor metal such as Cu or Al. Next, this is heated to about 120 ° C. to volatilize the organic solvent and at the same time, the particles of the Li alloy are diffusion-bonded to obtain a porous material. Diffusion bonding of the particles of the Li alloy can also be performed by volatilizing the organic solvent and then lightly pressing. The metal particle diameter of the raw material is preferably about 1 to 50 μm, and particularly preferably 5 to 20 μm from the viewpoint that the formed porous state is fine and dense and that the absorbability of a highly viscous electrolytic solution is not impaired.

[0012] The pressure casting method uses porous glass bead particles
This is a method of pressure-casting a raw material metal in a liquid phase into porous particles such as porous carbon particles. First, a volatile organic solvent is added to a predetermined amount of porous particles to impart fluidity to form a paste. This paste-like material is spread on the same conductor layer tape as above, and the organic solvent is volatilized by heating and drying.
Then, a liquid phase Li alloy is pressure-cast between the porous particles,
After crushing the porous particles by pressing the whole,
Ultrasonic cleaning is performed in a non-polar solvent such as hexane to remove the crushed porous particles to obtain a porous material.

The thermal spraying method is a method of forming a porous film by adjusting thermal spraying parameters such as powder particle size, thermal spraying distance, cooling rate and the like. The thermal spraying is plasma spraying, and the coating is formed in an atmospheric pressure atmosphere or a reduced pressure atmosphere of argon.

The conductor layer 5 is a current path also called a current collector depending on the shape of the battery, and the carbon layer 2 is formed on the conductor layer 5. The material of the conductor layer 5 is preferably a good conductor metal, such as Cu, Al, Au, Ag, N.
i etc. are illustrated.

Using the negative electrode for lithium secondary battery of the present invention
To form an excellent lithium secondary battery by
You can Positive electrode material constituting the positive electrode of the lithium secondary battery
Is not particularly limited, and is usually the positive electrode of a lithium secondary battery.
The positive electrode material used in₂O_Five, M
nO₂, LiMn₂O_Four, LiCoO₂, LiNi_0.5
Co_0.5O₂, LiNiO ₂, Li-Co-P complex acid
Compound (LiCo_0.5P_0.5O₂, LiCo_0.4P_{0. 6}O
₂, LiCo_0.6P_0.4O₂, LiCo_0.3Ni_0.3P
_0.4O₂, LiCo _0.2Ni_0.2P_0.6O₂Etc.), Ti
S₂, MoS₂, MoO₃A positive electrode material with an active material such as
Can be used. Among these, the rechargeable battery's electromotive force and charge / discharge
Li-Co-P compound that can increase the electric voltage in particular
A compound oxide can be preferably used.

In the present invention, the active material portion of the negative electrode is basically
The active material of the positive electrode, which contains Li as a positive electrode
Not containing Li (V₂O_Five, MnO₂, T
iS ₂, MoS₂, MoO₃Etc.) may be used, but L
The positive electrode active material containing i (LiCoO 2₂, LiNiO
₂, Li-Co-P-based composite oxide)
The amount of the negative electrode active material can be reduced. Also on
The positive electrode active material includes acetylene black and Ketjen.
Conductive materials such as black are also used in polytetrafluoroethylene.
A binder such as len or polyethylene is mixed.

As the electrolyte, an electrolytic solution in which salts are dissolved in an organic solvent or a solid electrolyte can be used. Liquid electrolyte,
That is, in the case of an electrolytic solution, this salt is LiCl
_{O 4, LiBF 4, LiPF 6} , LiAsF 6, LiA
lCl ₄ , Li (CF ₃ SO ₂ ) ₂ N and the like can be used, and ethylene carbonate, propylene carbonate, dimethyl sulfoxide, sulfolane, γ-butyrolactone, 1,
Prepared to a concentration of 0.1 to 3 mol / l by dissolving it in an organic solvent such as 2-dimethoxyethane, N, N-dimethylformamide, tetrahydrofuran, 1,3-dioxolane, 2-methyltetrahydrofuran, diethyl ether and a mixture thereof. Then used. 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, and derivatives, mixtures, complexes and the like for use. This solid electrolyte also serves as a separator for the negative electrode and the positive electrode.

[0018]

EXAMPLES The present invention will be described in more detail with reference to examples. Example 1 In this example, a specific manufacturing process of a negative electrode in which a porous layer made of a Li—Si alloy is formed on a carbon layer and which is used as an active material layer is shown, and further, the negative electrode is manufactured. An example of the cylindrical lithium secondary battery is shown.

[Production of Negative Electrode] Thickness 10 μm, Width 41 m
m, 300 mm in length on Cu tape, average particle size 5 μm
The natural graphite particles of 3 were developed using an organic solvent and the solvent was volatilized to form a carbon layer having a thickness of 80 μm. However, 1% by weight of PVDF was added and used as a binder for the natural graphite particles. A Li _3.5 Si alloy having a thickness of 5 μm is formed on the carbon layer by plasma spraying so as to exhibit porosity.
m to form a tape-shaped negative electrode (negative electrode tape).

[Preparation of Positive Electrode] Lithium carbonate, basic cobalt, and a phosphoric acid aqueous solution having a phosphoric acid content of 85% were used at a composition ratio (ratio of the number of component elements) of Li: Co: P = 2: 1.
The respective amounts of 5: 0.5 were weighed and sufficiently mixed, and then the mixture was put into 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. This mixture was crushed and adjusted to a powder having an average particle size of 20 μm by sieving. 46 parts by weight of this powder, 4 parts by weight of acetylene black, 2 parts by weight of polyvinylidene fluoride and 50 parts by weight of n-methylpyrrolidone were mixed, and this was 38 mm wide and 30 mm long.
It was applied to an aluminum tape having a thickness of 0 mm and a thickness of 20 μm and further vacuum dried to produce a tape-shaped positive electrode (positive electrode tape) having a thickness of 100 μm.

[Preparation of Electrolyte Solution] 1 mol / liter of lithium perchlorate was dissolved in propylene carbonate to prepare an electrolytic solution of a non-aqueous electrolyte.

[Assembly of Lithium Secondary Battery] A polypropylene separator is sandwiched between the negative electrode tape and the positive electrode tape to form three layers, which are wound and inserted into a battery can.
After injecting 3 ml of the electrolytic solution, the electrodes were connected and the battery can was sealed to obtain an AA-type (cylindrical) lithium secondary battery.

[Charge / Discharge Characteristics] The initial maximum values of the discharge capacity and electromotive voltage of the lithium secondary battery are 500 mAh,
It was 4.5 V, and it was confirmed that it had a large capacity and high electromotive force. Using this lithium secondary battery, a charging current of 100
It is charged for 1 hour until the electromotive force reaches 4.2 V at mA, and the electromotive force is 2.75 at the same current value as the charging current.
A charging / discharging cycle of discharging until V and resting for 1 hour was repeated 50 times. The discharge capacity retention rate after the charge / discharge cycle was 85%, and the discharge capacity per unit weight of the battery was 120 mAh / g.

Example 2 A lithium secondary battery was obtained in the same manner as in Example 1 except that the Li alloy forming the porous layer of the negative electrode in Example 1 was changed to Li _1.2 Zn alloy instead of Li _3.5 Si alloy. When manufactured and examined for charge and discharge characteristics similar to those of Example 1, the initial maximum values of discharge capacity and electromotive voltage of the lithium secondary battery were 520 mAh and 4.4 V, indicating that they had a large capacity and high electromotive force. The discharge capacity retention rate after charge / discharge cycles was 91%, and the discharge capacity per unit weight of the battery was 1
The open circuit voltage was 10 mAh / g and the voltage was 4.1V.

Comparative Example 1 Instead of the negative electrode tape in the above-mentioned Example 1 or 2,
A lithium secondary battery was manufactured in the same manner as in the above example except that only a 0.2 mm metal Li foil was used as the negative electrode tape, and the same charge / discharge cycle test was performed. The maintenance rate was 0%.

[0026]

As described in detail above, in the negative electrode for a lithium secondary battery of the present invention, carbon shares a large discharge capacity, and a porous layer made of a Li alloy shares a large electromotive force. By this action, it is possible to provide a lithium secondary battery having both a large discharge capacity and a high electromotive force. Furthermore, since the total surface area of the negative electrode in contact with the electrolytic solution is large, Li ions are dispersed and dendrite growth can be suppressed. This makes it possible to provide a lithium secondary battery that is less likely to deteriorate due to repeated charging and discharging.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing an example of a negative electrode for a lithium secondary battery of the present invention by partially enlarging it as a part of a battery.

[Explanation of symbols]

 A negative electrode for lithium secondary battery 1 active material layer 2 carbon layer 3 porous layer

Claims (4)

[Claims] 1. A negative electrode for a lithium secondary battery, comprising a laminated body formed by forming a porous layer made of a Li alloy on a carbon layer as an active material layer. 2. The Li alloy is a Li—Si alloy or Li—
The negative electrode for a lithium secondary battery according to claim 1, which is a Zn alloy. 3. The composition ratio (ratio of the number of component elements) of the Li—Si alloy is Li: Si = 1: (3.3 to 4.4),
The composition ratio of the Li—Zn alloy is Li: Zn = 1: (1-1.
The negative electrode for a lithium secondary battery according to claim 2, which is 5). 4. A lithium secondary battery comprising the negative electrode for a lithium secondary battery according to claim 1.