JP2005063805A - Anode and lithium secondary battery using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an anode for a lithium secondary battery with high capacity and yet small irreversible capacity. <P>SOLUTION: The anode for the lithium secondary battery is provided with an anode mixture layer containing an anode active material containing at least one kind selected from a group consisting of Si, Sn, and an Si-Ti group alloy, and forms a lithium layer so as to cover a surface of the anode mixture layer. The anode active material is preferable to have irreversible capacity at a first cycle of charge and discharge cycles. It is preferable to use the anode mixture layer satisfying a relational expression: Y=X/(1-a) (provided, 'a' denotes an irreversible capacity ratio) between a cathode capacity X per unit volume (mAh/sq.cm) and an anode capacity Y per unit volume (mAh/sq.cm). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a lithium secondary battery having a low irreversible capacity at the beginning of a charge / discharge cycle, and more particularly to a negative electrode including a negative electrode material made of a solid solution or an intermetallic compound effective as a negative electrode active material.

A lithium secondary battery using lithium or a lithium compound as a negative electrode material has a high voltage and a high energy density. Many studies have been conducted with the aim of further improving the performance of this battery.
Until now, as a positive electrode active material of a lithium secondary battery, transition metal such as LiCoO ₂ , LiMn ₂ O ₄ , LiFeO ₂ , LiNiO ₂ , V ₂ O ₅ , Cr ₂ O ₅ , MnO ₂ , TiS ₂ , MoS _2, etc. Oxide and chalcogen compounds have been proposed.

  On the other hand, various materials have been studied for the negative electrode, and carbon materials, aluminum alloys, and the like have been put into practical use as negative electrode active materials for practical batteries. At present, carbon materials have the highest performance and are widely used. However, since this carbon material has a specific gravity smaller than that of the positive electrode material, there is a problem that the proportion of the occupied volume inside the battery is increased. In addition, since it has already been put to practical use with a capacity close to the theoretical capacity, it is difficult to further increase the energy density using this carbon material. In order to increase the capacity of batteries in the future, it is indispensable to increase the capacity or increase the density using negative electrode materials other than carbon materials.

  As a negative electrode material having high capacity, tungsten oxide, lithium compound of iron oxide, niobium oxide, iron oxide, cobalt oxide, oxide of silicon containing lithium, oxide containing vanadium, tin, germanium, silicon, etc. There have been proposed composite oxides containing, amorphous oxides containing tin, lead, silicon, or the like.

  Besides these oxides, iron silicide, non-ferrous metal silicide made of transition metal, nickel silicide, manganese silicide, alloy material of Si or Sn and Fe or Ni, Si, Sn, Zn, etc. An intermetallic compound containing at least one of the above, a material made of particles containing both a phase of Si, Sn, etc. and a phase made of an intermetallic compound containing Si, Sn, etc. as one of constituent elements has been proposed.

However, all of these materials have a problem that the irreversible capacity is large in the initial charge / discharge.
In order to solve this problem, an electrode formation method in which the irreversible capacity is charged electrochemically in advance (for example, Patent Document 1) and a method of making up for the irreversible capacity by attaching metallic lithium to the negative electrode have been proposed. (For example, Patent Documents 2 to 5). However, when these methods are used, there is a new problem.

  In the electrode formation method, the amount of electricity according to the purpose can be formed by controlling the amount of electricity supplied. However, since the electrode is once charged and then reassembled as a battery, the work becomes complicated. Further, in the method of attaching metallic lithium, when an electrolytic solution is injected into the battery, metallic lithium and the negative electrode material are short-circuited inside the negative electrode, and lithium automatically moves. However, in this method, the amount of lithium movement may be insufficient depending on how the metallic lithium is attached or the form of the negative electrode, which may cause variations and safety problems.

Furthermore, in order to arrange metallic lithium thinly on the entire surface of the negative electrode, there is a method in which metallic lithium is arranged on the negative electrode at equal intervals and rolled together with the negative electrode to thin the metallic lithium. However, in this case, the density of the negative electrode may be partially non-uniform due to rolling.
When these solid solutions and intermetallic compounds are used as the negative electrode active material, if the positive electrode capacity is fixed, an irreversible capacity occurs during the first cycle charge, so the amount of lithium that returns to the positive electrode during the first cycle discharge decreases. . For this reason, there was a problem that the amount of lithium reversibly moving between the positive electrode and the negative electrode during subsequent charging / discharging decreased, that is, the battery capacity decreased.

When graphite is used as the negative electrode active material, the irreversible capacity in the first cycle is about 5% of the theoretical capacity. However, when the above solid solution and intermetallic compound are used, for example, the irreversible capacity is as large as about 30% of the theoretical capacity, and the battery capacity is greatly reduced.
As a method for solving such a problem, it is conceivable to increase the capacity of the positive electrode in advance by an irreversible capacity. However, in this method, when batteries of the same volume and the same size are compared, the capacity as a battery is reduced.
JP 11-31531 A JP-A-8-102363 Japanese Patent Laid-Open No. 10-83834 Japanese Patent Laid-Open No. 11-86847 JP 2002-75454 A

  Therefore, in order to solve the above-described conventional problems, the present invention provides a lithium secondary battery having a small irreversible capacity by uniformly forming a metal lithium layer for the irreversible capacity so as to cover the entire surface of the negative electrode mixture. An object of the present invention is to provide a negative electrode for use. Moreover, it aims at providing the lithium secondary battery excellent in charging / discharging cycling characteristics by using this negative electrode.

The negative electrode for a lithium secondary battery of the present invention includes a negative electrode mixture layer containing at least one negative electrode active material selected from the group consisting of Si, Sn, and Si-Ti alloys, A metal lithium layer is formed so as to cover the surface.
The negative electrode active material preferably has an irreversible capacity in the first charge / discharge cycle.

For the positive electrode capacity X (mAh / cm ² ) per unit area, the negative electrode capacity Y (mAh / cm ² ) per unit area is a relational expression:
Y = X / (1-a)
It is preferable to use a negative electrode mixture layer satisfying (where a is an irreversible capacity ratio and is represented by the following formula 1).
Irreversible capacity ratio a = 1− (discharge capacity at the first cycle (mAh / cm ² ) /
Charging capacity in the first cycle (mAh / cm ² )) (Formula 1)

Furthermore, the amount Z (g / cm ² ) of lithium in the metal lithium layer is expressed by the relational expression:
It is preferable that (2.59 × 10 ⁻⁴ aX / (1-a)) × 0.95 ≦ Z ≦ (2.59 × 10 ⁻⁴ aX / (1-a)) × 1.05 is satisfied.

The metal lithium layer is preferably formed so as to cover the surface of the negative electrode mixture layer by vapor deposition or sputtering.
The negative electrode mixture layer is preferably obtained by applying a paste containing a negative electrode active material on a substrate and then drying it.
It is preferable that the negative electrode mixture layer be obtained by applying a paste containing a negative electrode active material on a substrate, followed by drying and further rolling.

The negative electrode mixture layer is preferably formed on the substrate by vapor deposition, sputtering, or CVD.
It is preferable that the negative electrode mixture layer be obtained by further heat treatment after being formed on the substrate by vapor deposition, sputtering or CVD.
The present invention also relates to a lithium secondary battery comprising the above negative electrode, a positive electrode including a positive electrode active material, and a separator including an electrolytic solution.

  According to the present invention, a negative electrode having a high capacity and a small irreversible capacity can be provided. Moreover, the lithium secondary battery excellent in cycling characteristics can be provided by using this negative electrode.

The present invention relates to a negative electrode for a lithium secondary battery comprising a negative electrode mixture layer containing a negative electrode active material and a metallic lithium layer that uniformly covers the surface of the negative electrode mixture layer.
After inserting the positive electrode and the negative electrode described above via a separator into the battery case and injecting an electrolyte, a local battery is formed on the surface of the negative electrode mixture, and the surface of the negative electrode mixture and the negative electrode mixture Evenly covering the metallic lithium is short-circuited. For this reason, metallic lithium reacts with the solid solution or intermetallic compound that is the negative electrode active material in the negative electrode mixture, and is inserted into the negative electrode active material. At this time, since the metallic lithium exists so as to uniformly cover the negative electrode mixture surface, the metallic lithium is uniformly inserted into the negative electrode active material regardless of the position of the negative electrode.

  In addition, when metal lithium is inserted into the negative electrode active material, a portion where the metal lithium layer is present remains as a space. Due to the presence of this space, the expansion of the negative electrode accompanying the insertion of metallic lithium can be mitigated.

The negative electrode mixture layer includes, for example, a negative electrode active material, a conductive agent such as a carbon material, and a binder such as carboxymethylcellulose (CMC). The negative electrode mixture layer may be composed of only a negative electrode active material.
The negative electrode active material is preferably at least one selected from the group consisting of Si, Sn, and Si—Ti alloys.
As for Si, the crystal state is not particularly limited, but amorphous one is more preferable.
For the Si—Ti alloy, the composition ratio M (M = Si / Ti) of Si and Ti is preferably 2 <M ≦ 4. Further, it is preferable that the Si-Ti alloy consists of two phases TiSi ₂ alloy phase and the Si phase.

The negative electrode capacity Y (mAh / cm ² ) per unit area is related to the positive electrode capacity X (mAh / cm ² ) per unit area:
Y = X / (1-a)
It is preferable to use a negative electrode mixture layer satisfying (where a is an irreversible capacity ratio and is represented by the following formula 1).
Irreversible capacity ratio a = 1− (discharge capacity at the first cycle (mAh / cm ² ) /
Charging capacity in the first cycle (mAh / cm ² )) (Formula 1)

Further, the lithium amount Z (g / cm ² ) in the metal lithium layer is expressed by the relational expression:
(2.59 × 10 ⁻⁴ aX / (1-a)) × 0.95 ≦ Z ≦ (2.59 × 10 ⁻⁴ aX / (1-a)) × 1.05
It is preferable to satisfy.
That is, it is preferable that the lithium amount Z in the metal lithium layer is 95 to 105% of the lithium amount corresponding to the irreversible capacity. Here, 2.59 × 10 ⁻⁴ aX / (1-a) represents the amount of lithium corresponding to the irreversible capacity of the negative electrode.

The amount of lithium corresponding to this irreversible capacity is derived as follows.
From the above, the irreversible capacity (mAh / cm ² ) is expressed as Y × a = aX / (1-a). The lithium amount (g / cm ² ) corresponding to this irreversible capacity is expressed as ((aX / (1-a) × 3600) / (1000 × 96500)) × 6.939. By calculating this, the amount of lithium corresponding to the irreversible capacity is derived as 2.59 × 10 ⁻⁴ aX / (1-a). In addition, 96500 is a Faraday constant (amount of electricity per 1 mol of electrons), and 6.939 is an atomic weight of Li.

The metallic lithium layer is formed by a method such as vapor deposition or sputtering.
In addition, the thickness of the metal lithium layer covering the surface of the negative electrode mixture layer is not particularly limited, but if the metal lithium layer formed on the negative electrode surface is too thick, the metal lithium may not be sufficiently consumed when the electrolyte is injected. There is sex.

The negative electrode mixture layer is obtained by applying and drying a paste containing a negative electrode active material on a substrate. Further, the negative electrode mixture layer may be further rolled after the paste is applied on the substrate and dried. The paste may be either aqueous or organic.
The negative electrode mixture layer is formed on the substrate by a method such as vapor deposition, sputtering, or CVD. Furthermore, after forming the negative electrode mixture layer by these methods, heat treatment may be applied. The method for forming the negative electrode mixture layer is not particularly limited thereto.

The lithium secondary battery of this invention is comprised from said negative electrode, a positive electrode, and the separator containing electrolyte solution.
As the positive electrode active material, for example, it can be used lithium-cobalt composite oxide such as LiCoO _2, lithium-nickel composite oxide such as LiNiO _2, lithium-manganese composite oxide such as LiMn ₂ O _4.

Any electrolytic solution may be used as long as it is usually used in lithium secondary batteries.
As the electrolytic solution, a solvent containing a lithium salt is used. Examples of the lithium salt include lithium such as lithium hexafluorophosphate and lithium tetrafluoroborate. Moreover, as said solvent, organic solvents, such as ethylene carbonate, diethyl carbonate, propylene carbonate, dimethyl carbonate, or these mixed solvents are mentioned.
Examples of the separator include polyethylene, polypropylene, and nonwoven fabric.

Hereinafter, embodiments of the present invention will be described in detail.
In addition, work steps such as production of materials and battery assembly other than the production of the negative electrode in this example were all performed in a nitrogen atmosphere. The moisture content in the atmosphere was set to a dew point of −60 ° C., and the oxygen partial pressure was set to 50 ppm or less.

Example 1
(I) Production of negative electrode mixture An alloy or metal powder having a predetermined composition ratio was used as a starting material, and was melted in a vacuum melting furnace and then rapidly cooled to obtain a Ti-Si (composition ratio Si / Ti = 3) alloy.
Ti-Si alloy powder as the negative electrode active material obtained above, carbon powder as a conductive agent, carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) as a binder, a weight ratio of 90: The mixture was mixed in water (solid / liquid ratio 45:55) at a ratio of 10: 5: 2 to obtain a negative electrode paste. This negative electrode paste was applied onto a core material made of copper foil, dried at 60 ° C., and further vacuum dried at 110 ° C. for 10 hours. It was punched into a disk shape having a diameter of 13.0 mm to obtain a negative electrode mixture. At this time, the amount of the negative electrode active material was adjusted so that the negative electrode capacity Y satisfied the relational expression of Y = X / (1-a) with respect to the positive electrode capacity X and irreversible capacity ratio a described later. In addition, the irreversible capacity ratio a of this negative electrode active material was 0.18.

(Ii) Formation of a metallic lithium layer on the surface of the negative electrode mixture Lithium is vapor-deposited on the entire surface of the disk-shaped negative electrode mixture obtained above, and a predetermined thickness of the metallic lithium so as to cover the surface of the negative electrode mixture A layer was formed to obtain a negative electrode. At this time, metallic lithium was evaporated by heating and melting at 480 ° C. under 10 ⁻² Pa. The amount of metallic lithium was set to an amount corresponding to the irreversible capacity (a × Y) of the negative electrode mixture.

(Iii) Production of positive electrode Lithium cobaltate as a positive electrode active material, carbon powder as a conductive agent, and polytetrafluoroethylene powder as a binder were mixed in a weight ratio of 100: 5: 5, An agent was obtained. This was rolled into a sheet and punched into a disk having a diameter of 12.5 mm to make a positive electrode. At this time, the amount of the positive electrode active material was adjusted so that the positive electrode capacity X was about 3 mAh / cm ² .

(Iv) Assembly of lithium secondary battery A 2016-size coin-type lithium secondary battery having the structure shown in FIG. 1 was produced using the positive electrode and the negative electrode obtained above. The procedure for manufacturing this battery is shown below.
First, the positive electrode 15 was pressure-bonded to the current collector 16 bonded to the case 14. Next, a separator 13 made of a porous polyethylene sheet was placed on the positive electrode 15, and then an electrolyte solution was injected into the case 14. At this time, an electrolytic solution in which lithium hexafluorophosphate was dissolved at a concentration of 1M in a solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1: 1 was used. The negative electrode 12 was pressure-bonded to the current collector 18 on the inner surface, and the sealing plate 11 provided with the sealing ring 17 at the peripheral edge was combined with the case 14. Subsequently, it sealed using the press sealing machine. This battery was designated as battery A.

Example 2
A negative electrode mixture was prepared in the same manner as in Example 1 except that Si was used as the negative electrode active material. The irreversible capacity ratio of this negative electrode active material was 0.13. In the same manner as in Example 1, a metal lithium layer was formed on the surface of the negative electrode mixture to obtain a negative electrode. And the battery B was produced by the method similar to Example 1 except having used this negative electrode.

Example 3
A negative electrode active material composed of two phases of TiSi ₂ phase and Si phase was produced in the same manner as in Example 1. The weight ratio between the TiSi ₂ phase and the Si phase was 80:20.
A negative electrode mixture was prepared in the same manner as in Example 1 except that the alloy obtained above was used as the negative electrode active material. The irreversible capacity ratio of this negative electrode active material was 0.10.
In the same manner as in Example 1, a metal lithium layer was formed on the surface of the negative electrode mixture to obtain a negative electrode. A battery C was produced in the same manner as in Example 1 except that this negative electrode was used.

Example 4
A Si layer was formed as a negative electrode active material on a substrate made of copper foil by a CVD method (chemical vapor deposition method) using a predetermined source gas and a hydrogen carrier gas. The irreversible capacity ratio of this Si layer was 0.04.
A metallic lithium layer was formed on the surface of the Si layer by the same method as in Example 1 to obtain a negative electrode. And the battery D was produced by the method similar to Example 1 except having used this negative electrode.

Example 5
An Sn layer as a negative electrode active material was formed on a substrate made of copper foil by a CVD method (chemical vapor deposition method) using a predetermined source gas and a hydrogen carrier gas. The irreversible capacity ratio of this Sn layer was 0.07.
In the same manner as in Example 1, a metallic lithium layer was formed on the surface of this Sn layer to obtain a negative electrode. A battery E was produced in the same manner as in Example 1 except that this negative electrode was used.

<< Comparative Example 1 >>
A battery F was produced in the same manner as in Example 1 except that the metal lithium layer was not formed on the surface of the negative electrode mixture.

[Evaluation]
For the batteries A to E of Examples 1 to 5 and the battery F of Comparative Example 1 obtained above, the battery voltage was measured after injection. And the following charge / discharge cycle test was conducted.
A constant current at a current density of 0.5 mA / cm ^2, was charged until the battery voltage reached 4.0V, a constant current at a current density of 0.5 mA / cm ^2, to discharge until the battery voltage reached 2.7V The cycle was repeated. Then, the charge capacity and discharge capacity and charge / discharge efficiency in the first cycle of each battery were examined. In addition, charging / discharging efficiency was obtained by the following formula.
Charge / discharge efficiency (%) = (discharge capacity / charge capacity) × 100

  The charge capacity and discharge capacity and charge / discharge efficiency in the first cycle of each battery are shown in FIGS. 2 and 3, the left side of the bar graph shown for each battery shows the charge capacity of the first cycle, and the right side shows the discharge capacity of the first cycle. Moreover, the plot shown with respect to each battery shows charging / discharging efficiency.

  In batteries A to E of Examples 1 to 5, it is considered that the open circuit voltage before charging is about 2.7 V and the negative electrode potential is lowered. Moreover, both the charge capacity and the discharge capacity were values close to the positive electrode capacity, and the charge / discharge efficiency was a value close to 100%.

  On the other hand, in the battery F of Comparative Example 1, the open circuit voltage before charging / discharging was about 0.15V. Further, the charge / discharge efficiency was the same as the value represented by (1−irreversible capacity ratio) × 100. Thus, by forming a lithium layer on the surface of the negative electrode mixture, it is possible to supplement the capacity of the negative electrode for the irreversible capacity, and the positive electrode capacity can be effectively used almost 100% after the second cycle. is there.

Example 6
A negative electrode mixture was prepared in the same manner as in Example 1. And the amount of lithium in the metallic lithium layer formed on the surface of the negative electrode mixture was varied to obtain negative electrodes having different lithium amounts. That is, a metallic lithium layer is formed on the surface of each negative electrode mixture in the range of 0.126 to 0.154 mg / cm ² so that the amount of lithium is 90%, 95%, 105% and 110% with respect to the irreversible capacity. Each was formed.
And coin type battery GJ was produced by the method similar to Example 1 except having used each negative electrode obtained above. Each battery was subjected to the same charge / discharge cycle test as described above.

FIG. 4 shows the charge capacities of the first and second cycles, the discharge capacity of the first cycle, and the charge / discharge efficiency of the batteries G to J and the battery A of Example 1. In the bar graph shown for each battery, the left side shows the charge capacity of the first cycle, the center shows the discharge capacity of the first cycle, and the right side shows the charge capacity of the second cycle. Moreover, the plot shown with respect to each battery shows charging / discharging efficiency.
In the case of the batteries G and H in which the metal lithium layer formed on the negative electrode mixture was less than the irreversible capacity, the charge capacity at the first cycle was the same as the positive electrode capacity, but the discharge capacity was lowered.

In addition, although the charge capacity at the second cycle is considered to decrease originally, Li _x CoO ₂ which is the positive electrode active material is calculated based on the amount of lithium in the range of x value of 0.5 ≦ x ≦ 1. In this charge / discharge test, the x value after the second cycle charge is considered to be less than 0.5. When the positive electrode active material is used up to x <0.5, the positive electrode potential is increased, so that the electrolytic solution is easily decomposed and gas such as CO ₂ is generated. For this reason, in the battery G in which the amount of lithium on the surface of the negative electrode mixture was 90% with respect to the irreversible capacity, the subsequent cycle characteristics deteriorated.

  On the other hand, when the amount of lithium on the surface of the negative electrode mixture was larger than the irreversible capacity, the charge capacity and discharge capacity at the first cycle were smaller than the positive electrode capacity. This is because metal lithium corresponding to the excess of the irreversible capacity was used for the reversible reaction in the negative electrode, and thus an amount of lithium corresponding to this excess remained on the positive electrode after charging. From the above, it was found that the amount of lithium formed on the negative electrode mixture preferably corresponds to a capacity in the range of 95 to 105% with respect to the irreversible capacity.

  Since the negative electrode for a lithium secondary battery of the present invention has a high capacity and a small irreversible capacity, it can be applied to a lithium secondary battery that requires excellent cycle characteristics.

It is a schematic longitudinal cross-sectional view of the coin-type lithium secondary battery of this invention. It is a figure which shows the charging / discharging characteristic in the battery A of Example 1 of this invention, and the battery F of the comparative example 1. FIG. It is a figure which shows the charging / discharging characteristic in battery AE of Examples 1-5 of this invention. It is a figure which shows the charging / discharging characteristic in the battery A of Example 1 of this invention, and the batteries GJ of Example 6. FIG.

Explanation of symbols

11 Sealing Plate 12 Negative Electrode 13 Separator 14 Case 15 Positive Electrode 16 Current Collector 17 Sealing Ring 18 Current Collector

Claims (10)

  A negative electrode mixture layer containing at least one negative electrode active material selected from the group consisting of Si, Sn, and Si—Ti alloys is provided, and a metallic lithium layer is formed so as to cover the negative electrode mixture layer The negative electrode for lithium secondary batteries characterized by the above-mentioned.   2. The negative electrode for a lithium secondary battery according to claim 1, wherein the negative electrode active material has an irreversible capacity in a first cycle of a charge / discharge cycle. The capacity Y (mAh / cm ² ) of the negative electrode per unit area with respect to the capacity X (mAh / cm ² ) of the positive electrode per unit area is a relational expression:
Y = X / (1-a)
2. The negative electrode for a lithium secondary battery according to claim 1, wherein a negative electrode mixture layer satisfying (where a is an irreversible capacity ratio and is represented by the following formula 1) is used.
Irreversible capacity ratio a = 1− (discharge capacity at the first cycle (mAh / cm ² ) /
Charging capacity in the first cycle (mAh / cm ² )) (Formula 1) Furthermore, the amount Z (g / cm ² ) of lithium in the metal lithium layer is expressed by the relational expression:
(2.59 × 10 ⁻⁴ aX / (1-a)) × 0.95 ≦ Z ≦ (2.59 × 10 ⁻⁴ aX / (1-a)) × 1.05
The negative electrode for a lithium secondary battery according to claim 3, wherein:   2. The negative electrode for a lithium secondary battery according to claim 1, wherein the metal lithium layer is formed so as to cover the surface of the negative electrode mixture layer by vapor deposition or sputtering.   2. The negative electrode for a lithium secondary battery according to claim 1, wherein the negative electrode mixture layer is obtained by applying a paste containing a negative electrode active material on a substrate and then drying the paste.   2. The negative electrode for a lithium secondary battery according to claim 1, wherein the negative electrode mixture layer is obtained by applying a paste containing a negative electrode active material on a substrate, drying, and rolling the paste.   2. The negative electrode for a lithium secondary battery according to claim 1, wherein the negative electrode mixture layer is formed on a substrate by vapor deposition, sputtering or CVD.   2. The negative electrode for a lithium secondary battery according to claim 1, wherein the negative electrode mixture layer is obtained by further heat treatment after being formed on a substrate by vapor deposition, sputtering or CVD.   A lithium secondary battery comprising the negative electrode according to claim 1, a positive electrode containing a positive electrode active material, and a separator containing an electrolytic solution.

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