JPH10162823A - Non-aqueous secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a non-aqueous secondary battery of a high capacity, which is excellent in cycle characteristics. SOLUTION: In a non-aqueous secondary battery having a positive electrode, a negative electrode and a non-aqueous electrolyte, the negative electrode is made of a silicon alloy or a tin alloy. The silicon alloy is preferably an alloy of Si and Fe or Si and Ni, wherein the content of Si in the silicon alloy is preferably 50mol% or more. The tin alloy is preferably an alloy of Sn and Fe or Sn and Ni, wherein the content of Sn in the tin alloy is preferably 50mol% or more.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a non-aqueous secondary battery, and more particularly, to a non-aqueous secondary battery having a high capacity and excellent cycle characteristics.

[0002]

2. Description of the Related Art Non-aqueous secondary batteries typified by lithium secondary batteries have high capacity, high voltage, and high energy density, and therefore have great expectations for their development.

In this non-aqueous secondary battery, an organic solvent-based electrolytic solution in which a lithium salt is dissolved in an organic solvent has been used, and lithium or a lithium alloy has been used as a negative electrode active material. In this case, a high capacity can be expected, but there is a problem that an internal short circuit is apt to occur due to the growth of lithium dendrite at the time of charging, which deteriorates battery characteristics and lacks safety.

Therefore, it has been studied to use a carbon material such as activated carbon or graphite capable of doping and undoping lithium ions as a negative electrode active material instead of lithium or a lithium alloy (Japanese Patent Publication No. 4-24831). And Japanese Patent Publication No. 5-17669).

[0005] The above graphite can capture one lithium ion per six carbon atoms, which corresponds to 830 mAh / ml in terms of capacity per unit volume.

However, this graphite is fully charged (372 mAh / g) due to the entrance and exit of lithium ions due to charge and discharge.
At the time of substantial lithium (state containing lithium), the interlayer distance is increased by about 10% compared to that at the time of complete discharge (state not including lithium). When charging and discharging are repeated, the crystal collapses due to the expansion and contraction, and the characteristics are deteriorated. . Therefore, to obtain a life of 500 cycles or more with graphite, usually 250 mAh / g
(600 mAh / ml) or less.

[0007] As one having a higher capacity than graphite, there is low-crystalline carbon. This low-crystalline carbon has a carbon-carbon bond distance of about 20% longer than graphite, so that the amount of lithium inserted can be increased, and there is almost no expansion or contraction of the lattice spacing during charging and discharging. Life expectancy is expected to be longer.

However, this low crystalline carbon has a theoretical maximum of 12
Although a high capacity of up to 00 mAh / g (that is, a state of C ₂ Li) can be expected, only about 800 mAh / g has been actually developed.

[0009] From the viewpoint that high capacity can be expected, the use of lithium alloys (including intermetallic compounds) as the negative electrode active material is still being studied. A typical example is a Li-Al alloy. In this Li-Al alloy, a Li-Al alloy is formed using a metal-bonded Al-Al skeleton as a matrix, and Li-Al alloy from the Li-Al alloy is formed.
When the charge and discharge are repeated, the Li-Al alloy is pulverized and the negative electrode swells and the electrolyte is not recharged. There is a problem that necessary absorption is caused and characteristics are deteriorated. In addition to the Li-Al alloy, a Li-Pb alloy, a Li-Sb alloy, and the like have been proposed, but these also show the same deterioration tendency as the Li-Al alloy.

Also, Mg ₂ S, which has higher ionicity than the alloy,
The same deterioration occurs in a compound of Li with n, Mg ₂ Si, Si, Sn, Ge or the like, and a high capacity and long life negative electrode material has not been obtained.

Further, attempts have been made to use metal oxides for the purpose of increasing capacity density and improving cycle characteristics. This uses SiO or SnO as a starting material for the negative electrode. The first chemical conversion reaction is as shown in the following formula (1).

SiO + 6Li ⁺ + 6e ⁻ → SiLi ₄ + Li ₂ O (1)

[0013] That is, with respect to 1 mol of SiO, 6 equiv Li ⁺ and e ^- SiL (electrons) reacts, the active material
i ₄ and Li ₂ O which does not contribute to the charge / discharge reaction are generated.

As is apparent from the above formula (1), the chemical formation requires 6 equivalents of Li ⁺ and e ⁻ , of which 2
The equivalent is consumed to produce Li ₂ O. Therefore, the active material generation efficiency is only 67% at the maximum. Usually the second
Since 100% of SiLi ₄ is not used in the second charge / discharge, the initial charge efficiency is only about 50%.

Li ⁺ used for chemical conversion is LiCo of the positive electrode.
Since it is supplied from O ₂ , LiMn ₂ O _4, etc., when the irreversible capacity is large as described above, the battery capacity becomes small.

[0016]

As described above, Li-
Al, Li-Pb, Mg ₂ Si (Li _x ) and the like have a high capacity, but have poor cycle characteristics, and metal oxides such as SiO and SnO have good cycle characteristics, but have a large irreversible capacity during chemical formation. When assembled, lithium ions in the positive electrode are wasted.

An object of the present invention is to solve the above-mentioned problems of the prior art, and to provide a non-aqueous secondary battery having a high capacity and excellent cycle characteristics.

[0018]

According to the present invention, a positive electrode during chemical formation is formed by using an alloy of a metal that does not alloy with Li, such as Fe or Ni, among silicon alloys such as Si ₂ Fe or Si ₂ Ni. Increase the efficiency of lithium ion use,
In addition, by providing a skeletal structure that does not form an alloy with lithium such as Fe or Ni even during the cycle, cycle deterioration due to pulverization and agglomeration of active materials (SiLi ₄ and SnLi ₄ ) generated with the cycle is prevented, The above object has been achieved.

The above formula (1) is a reaction for forming SiLi ₄ , and the composition of the alloy of Si and Li is not only SiLi ₄ but also SiLi _x (0 ≦ x ≦ 5). In the description, it illustrated only SiLi ₄ of the typical composition, in addition to this SiLi _4, the SiL
such as _{i x (0 ≦ x ≦ 5} ) belonging SiLi ₂ or SiLi including.

Therefore, the above-mentioned chemical conversion reaction will be described first. Taking Si ₂ Fe as an example of a silicon alloy and showing its chemical reaction, the following formula (2) is obtained.

Si ₂ Fe + 8Li ⁺ + 8e ⁻ → 2SiLi ₄ + Fe (2)

As shown in the above formula (2), 1 mol of Si
Equivalents of Li ⁺ and e ⁻ to ₂ Fe are consumed, and 2 mol of SiLi ₄ and 1 mol of Fe are produced. The active material that is effective for the battery is SiLi ₄ , and the 8 equivalents of Li ⁺ consumed on the left side of the above formula (2) are all used for generating the active material.

In the case of the above-mentioned metal oxides such as SiO and SnO, one-third of the used Li ⁺ is consumed for producing Li ₂ O which does not contribute to the charge / discharge reaction. In some cases, all of the Li ⁺ used is used to generate the active material, improving the utilization of lithium ions.

Next, the cycle characteristics will be described. In the case of the present invention, since the matrix (skeleton) is Fe, Al, Pb, Mg and the like in the case of conventional Li-Al, Li-Pb, Mg ₂ Si, etc. Unlike this, it is stable because it does not form an alloy with Li.

Therefore, even if SiLi ₄ of the active material is pulverized or agglomerated in the charge / discharge cycle, the change remains in a minute portion and the entire electrode is not deformed. As a result, the cycle characteristics are stable without deterioration.

[0026]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, a silicon alloy used for a negative electrode is composed of Si and Li such as Fe, Ni, W and Mo.
And a metal that does not form an alloy. Specific examples of such a silicon alloy include Si, as described above.
_In addition to 2Fe, for example, Si ₂ Ni, SiFe, Si
Ni, SiW, SiMo and the like can be mentioned.

The tin alloy is also an alloy of Sn and a metal which does not form an alloy with Li, such as Fe or Ni. Specific examples of such a tin alloy include, for example, Sn ₂ Fe, S
n ₂ Ni and the like.

These alloys also have the above-mentioned Si ₂ Fe
The same effect as in the case of can be expected. For example, Si ₂ N
Also in the case of i, the chemical conversion reaction is as follows ^: Si ₂ Ni + 8Li ⁺ + 8e ⁻ → 2SiLi ₄ + Fe (3) As shown in the following formula (3), 8 equivalents of Li ⁺ and e ⁻ are added to 1 mol of Si ₂ Ni. Consumed to produce 2 moles of SiLi ₄ and 1 mole of Ni,
SiLi ₄ acts as an active material, and Ni acts as a matrix during charge / discharge cycles. Then, 8 equivalents of Li ⁺ consumed in this chemical conversion reaction are all active materials (Si
Li ₄ ) is used to generate lithium ions, thereby improving the utilization rate of lithium ions. Ni, which forms a matrix in a charge / discharge cycle, does not form an alloy with Li, so that the cycle characteristics do not deteriorate and become stable.

Also, in the case of tin alloys such as Sn ₂ Fe and Sn ₂ Ni, the chemical reaction is carried out according to the following equations (4) and (5): Sn ₂ Fe + 8Li ⁺ + 8e ⁻ → 2SnLi ₄ + Fe (4) Sn ₂ Ni + 8Li ⁺ + 8e ⁻ → 2SnLi ₄ + Fe (5) 8 equivalents of Li with respect to 1 mol of Sn ₂ Fe or Sn ₂ Ni
⁺ And e ^- are consumed, 2 mol of SnLi ₄ and 1 mol of F
e and Ni are generated, SnLi ₄ acts as an active material,
Fe and Ni act as a matrix during a charge / discharge cycle. Then, 8 equivalents of L consumed in this conversion reaction
i ⁺ is used to generate active material (SnLi ₄ ),
Since the utilization rate of lithium ions is improved and Fe or Ni serving as a matrix in a charge / discharge cycle does not form an alloy with Li, the cycle characteristics are stable without deterioration.

In the above silicon alloy, Si is 50
It is preferably at least mol%. This is because the amount of Li that reacts by increasing the ratio of Si can be increased, and the capacity can be increased. Also in the tin alloy, Sn is preferably at least 50 mol%. This also increases the amount of Li that reacts by increasing the ratio of Si, as in the case of the silicon alloy,
This is because the capacity can be increased.

In the present invention, as the positive electrode active material, various materials can be used without any particular limitation. In particular, lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide (these are usually , And LiCoO ₂ , LiMn ₂ O ₄ , and LiNiO ₂ , respectively, and the ratio of Li to Co, the ratio of Li to Mn, and the ratio of Li to Ni often deviate from the stoichiometric composition. Is preferably used.

The positive electrode is formed by adding a conductive auxiliary such as phosphorus (scale) graphite, acetylene black or carbon black to the positive electrode active material, if necessary, for example, polyvinylidene fluoride, tetrafluoroethylene, or the like.
The positive electrode mixture prepared by adding a binder such as ethylene propylene diene terpolymer is pressed or formed into a paste by adding a solvent, and the paste is formed into a metal foil (for example, aluminum foil, titanium foil, platinum foil, etc.).
It is produced through a process of applying and drying on a current collector made of such as. However, the method for producing the positive electrode is not limited to the above-described example.

The negative electrode contains, for example, a mixture of the above-mentioned silicon alloy or tin alloy, or an alloy thereof, a conductive auxiliary such as phosphorus (scale) graphite, acetylene black, carbon black, and a binder. Make an electrode body,
It is assembled in a battery and is produced by a chemical reaction at the time of the first charging after the battery is assembled. However, the anode may be finished outside the battery in advance without using a method of performing a chemical reaction inside the battery.
As the shape of the negative electrode, a coin-shaped battery, a button-shaped battery, a pellet-shaped one produced through a step of pressure-molding a negative electrode mixture of the above composition, and a cylindrical battery or a prismatic battery are used. In many cases, a sheet is prepared by adding a solvent or the like to the negative electrode mixture having the above composition to prepare a paste, applying the paste to a copper foil or a nickel foil, and drying the paste. However, the method for manufacturing the negative electrode and the form thereof are not limited to the above examples.

As the non-aqueous electrolyte, any of a liquid electrolyte using an organic solvent and a solid electrolyte such as a polymer electrolyte can be used. As the above liquid electrolyte, that is, as an electrolytic solution, for example, 1,2-dimethoxyethane, 1,2-diethoxyethane, propylene carbonate, ethylene carbonate, γ-butyrolactone, tetrahydrofuran, 1,3-dioxolan, diethylene carbonate, dimethyl In a single solvent or a mixed solvent of two or more kinds such as carbonate and ethyl methyl carbonate, for example, LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiCl
An organic solvent-based electrolyte prepared by dissolving solutes such as O ₄ , LiPF ₆ , and LiBF ₄ alone or by dissolving two or more thereof is used.

[0035]

Next, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to only these examples.

Example 1 100 mg of Si ₂ Fe powder having an average particle diameter of 15 μm was press-formed and formed into a disk having a diameter of 16 mm and a thickness of 0.1 mm.

For the positive electrode, 10 mg of phosphorous graphite and 10 mg of polytetrafluoroethylene were mixed with 200 mg of LiCoO ₂ powder having an average particle size of 6 μm, and the mixture was pressed and molded to a diameter of 16 μm.
What was formed into a disk shape having a thickness of 1.0 mm and a thickness of 1.0 mm was used.

As a separator, a laminated polyethylene porous thin film and a polyethylene nonwoven fabric are used. As a non-aqueous electrolyte, LiPF ₆ is mixed with a mixed solvent of ethylene carbonate and methyl ethyl carbonate at a volume ratio of 1: 1.
Using a liquid electrolyte prepared by dissolving 1 mol / liter, ie, an organic solvent-based electrolyte, sandwiching the separator between the positive electrode and the negative electrode, pressing the positive electrode, the separator, and the negative electrode, The battery is charged at 3 mA for 20 hours, and thereafter, the discharge is performed at 3 mA to 3.0 V, and the charge is performed at 3 mA.
And repeated constant current charging and discharging up to 4.2V.

Comparative Example 1 50 mg of Si powder was used instead of Si ₂ Fe used in Example 1.
A non-aqueous secondary battery was produced in the same manner as in Example 1 except that a negative electrode was formed by pressing into a disk having a diameter of 16 mm and a thickness of 0.1 mm. Charge and discharge were performed under the conditions.

Comparative Example 2 Instead of Si ₂ Fe used in Example 1, SiO ₂ powder 10
0mg is press molded and 16mm in diameter, 0.2mm in thickness
A non-aqueous secondary battery was produced in the same manner as in Example 1 except that the disk-shaped product was used as the negative electrode, and charged and discharged under the same conditions as in Example 1.

FIG. 1 shows the state of the first charging and the first discharging of the batteries of Example 1 and Comparative Examples 1 and 2. FIG. 2 shows the cycle characteristics of the batteries of Example 1 and Comparative Examples 1 and 2.

As shown in FIG. 1, in the first embodiment, about 60 mAh of electricity is charged in the first discharge, and about 6 mAh is charged in the first discharge.
An electric quantity of 0 mAh can be taken out. Even after the second time, as shown in FIG. 2, the charge / discharge amount was about 60 mAh, showing stable cycle characteristics, and the cycle characteristics were excellent.

On the other hand, in Comparative Example 1, the initial charge amount is about 60 mAh, which is almost the same as that of Example 1, but the discharge amount is reduced to about 40 mAh, and when the cycle is repeated thereafter, as shown in FIG. The capacity becomes extremely small. This is considered to be because SiLi ₂ is pulverized in the negative electrode.

In Comparative Example 2, the initial charge amount was about 60 m.
Ah was almost the same as Example 1, but the initial discharge amount was about 40 mAh, which was smaller than about 60 mAh in Example 1. However, after the second time, the charge / discharge amount was stable at about 40 mAh as shown in FIG. In the case of Comparative Example 1, Si
Although cycle deterioration occurred by pulverization of Li ₂ , Comparative Example 2
In the case of the first charge, Li ₂ O is generated by the first charge, and this Li ₂ O
Has a good effect on the cycle characteristics, and is presumed to be due to suppression of cycle deterioration.

As described above, Example 1 had a high capacity because SiLi _x and Fe were generated during the first charge, and electricity was not required to generate Fe. Therefore, about 60 mAh was charged. It is estimated that about 60 mAh was able to be discharged.

The reason that the cycle deterioration of Example 1 did not occur is that even when Fe performed the function of Li ₂ O generated in Comparative Example 2 and SiLi _x was pulverized, the charge / discharge characteristics of the battery were adversely affected. It is thought that this is because the occurrence of

[0047]

As described above, according to the present invention,
A non-aqueous secondary battery having high capacity and excellent cycle characteristics can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram showing initial charge / discharge behaviors of batteries of Example 1 and Comparative Examples 1 and 2.

FIG. 2 is a diagram showing cycle characteristics of batteries of Example 1 and Comparative Examples 1 and 2.

Claims (5)

[Claims] 1. A non-aqueous secondary battery having a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode contains a silicon alloy or a tin alloy. 2. The non-aqueous secondary battery according to claim 1, wherein the silicon alloy is an alloy composed of Si and Fe or an alloy composed of Si and Ni. 3. The non-aqueous secondary battery according to claim 1, wherein the tin alloy is an alloy composed of Sn and Fe or an alloy composed of Sn and Ni. 4. The non-aqueous secondary battery according to claim 1, wherein the silicon content of the silicon alloy is 50 mol% or more. 5. A tin alloy containing 50 mol% of Sn.
The non-aqueous secondary battery according to claim 1 or 3, which is as described above.