JPH10294112A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate a problem in safety such as firing, and enhance discharge capacity per the volume by arranging a lithium oxide composed of LiCoO2 in a positive electrode, and specifying the composition of a negative electrode material arranged in a negative electrode of a lithium secondary battery constituted by filling a nonaqueous electrolyte. SOLUTION: A material having the composition shown by M100-x Six (x>=50 a t%) is used as a negative electrode material of a lithium secondary battery. In this case, M is composed of at least one kind of element selected from Ni, Fe, Co and Mn. When a quantity of Si exceeds 50 a t%, a crystal of Si easily deposits. Since it is difficult to be industrially manufactured when the quantity of Si becomes 90 a t% or more, the quantity of Si is preferably set to a range of 55 to 85 a t%. Since the area in which Li and Si contact with each other can also be expanded by forming metallic silicide as a fine crystal, chemical reaction can be efficiently made to progress.

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 particularly to a negative electrode material most suitable for increasing the capacity.

[0002]

2. Description of the Related Art With the rapid progress of semiconductor technology, recent electronic devices, especially personal computers, mobile phones,
AV equipment and the like have become smaller, lighter, and multifunctional, and at the same time, convenience has been greatly improved. The demand for high energy density, long life, light weight, and the like is increasing for the secondary batteries used in these batteries. Conventionally, nickel-cadmium batteries have been mainly used as small secondary batteries for consumer use, but in the 1990s, nickel-metal hydride batteries were commercialized in earnest.
In addition, remarkable technological innovations have been made such as the entry of the lithium secondary battery into the secondary battery market.

[0003] To summarize the characteristics of lithium batteries,
Higher energy density than conventional batteries. Low self-discharge. The operable temperature range is as wide as -20 to + 45 ° C. No memory effect. And so on. Due to these advantages, nickel-cadmium batteries and nickel-hydrogen batteries are currently the mainstream in the secondary battery market, but lithium secondary batteries are expected to replace them in the near future.

[0004] Lithium cobalt oxide (LiCoO ₂ ), which can provide a high discharge voltage, is used as the positive electrode material of currently marketed lithium secondary batteries, and carbon is used as the negative electrode material. The electrode material is made into a powder form, mixed with a conductive binder, and applied to a current collector to form an electrode. Aluminum foil is used for the current collector of the positive electrode, and copper foil is used for the current collector of the negative electrode. A porous film made of polyethylene or the like is used as a separator that insulates the positive electrode and the negative electrode inside the battery. In an actual battery, a positive electrode, a separator, and a negative electrode are placed in a cylindrical tube in a stacked state and wound in a roll. Furthermore, supporting electrolytes (LiClO ₄ , LiPF ₆ , LiBF
₄ )) is filled with a non-aqueous organic electrolyte.

[0005] In the lithium secondary battery configured as described above, a part of lithium in the positive electrode material LiCoO ₂ is released as ions into the organic electrolyte, and accordingly, the supporting electrolyte (dissolved in the organic electrolyte). The lithium ion of LiClO ₄ , LiPF ₆ , LiBF _{4 and the} like is inserted between carbon layers of the negative electrode material, whereby a charging operation is performed. On the other hand, at the time of discharging, lithium ions occluded in the negative electrode are released, and the generated electrons flow to an external circuit to generate electric energy. The reaction described above is shown in the following formula. The direction of the arrow in the formula indicates that the chemical reaction proceeds to the right during charging and conversely to the left during discharging. _{LiCoO 2 ⇔ Li 1-n CoO} 2 + nLi ++ ne- ( positive electrode) (1) C + nLi ++ ne- ⇔ Li n C ( negative electrode) (2) Accordingly, the reaction during charging and discharging the entire battery combined positive and negative electrodes The formula is LiCoO ₂ + C ⇔ Li _1-n CoO ₂ + LinC (3).

[0006] The chemical reaction process of the formulas (1) to (3) will be described below with reference to the movement of lithium ions. That is, at the time of charging, lithium in the LiCoO ₂ of the positive electrode becomes ions and moves into the electrolyte, and the lithium ions in the electrolyte are occluded by the carbon of the negative electrode and accumulated in a state of lithium ions. At the time of discharge, the opposite reaction occurs, and lithium ions in the carbon of the negative electrode material move into the electrolyte, and the lithium ions in the electrolyte are occluded in the positive electrode material.
LiCoO ₂ As described above, since the active materials of the positive electrode and the negative electrode release or occlude and release lithium ions to perform charge and discharge, the materials used for the positive electrode and the negative electrode need to have a material capable of efficiently performing this chemical reaction. is there.

[0007]

At present, a carbon electrode is most often used as a negative electrode of a lithium secondary battery.
When carbon is used for the negative electrode, it is charged and discharged rapidly,
Lithium ions in the electrolyte may be deposited as metallic lithium in a dendritic manner even on the surface of the carbon, causing an internal short circuit and leading to a reduction in capacity. In addition, since a lithium battery has a higher energy density than other secondary batteries, it is necessary to refrain from using a material having a problem in safety such as ignition. For this reason, there is a need for a material that can replace flammable carbon. Further, the discharge capacity per volume is more advantageous as the density is higher, but it is difficult to obtain a high discharge capacity because carbon has a density of 2.2 g / ml, which is about several times lower than that of a metal material.

On the other hand, carbon as a negative electrode material stores lithium ions between crystal layers at the time of charging, so that the charging capacity depends on the lithium capacity. However, the carbon capacity is limited. When lithium is contained in the carbon-based negative electrode at the maximum, the negative electrode becomes LiC ₆ . At this time, the capacity per weight is relatively large at 370 mAh / g, but the density of these carbon materials is as small as about 2.2 g / ml, and the capacity per volume is limited to about 700 mAh / ml. . For this reason, in order to improve the capacity of a standardized secondary battery, it is necessary to develop a negative electrode material having a capacity of 700 mA-h / ml or more.

Therefore, as disclosed in Japanese Patent Application Laid-Open No. 7-240201, there is a proposal to apply an alloy material to a negative electrode material. It discloses the application of a non-ferrous metal silicide composed of a transition element, for example, CoSi _2-3 , Mn ₂ Si, Mo ₃ Si, NiSi, WSi _{2 and the} like. Further, in the battery debate 1995, a presentation to an improvement in discharge characteristics of NiSi ₂ by intermetallic compounds in the "lithium secondary battery electrode characteristics of ZnS type · CaF ₂ -type structure intermetallic compound".

[0010] This article describes that an intermetallic compound of NiSi ₂ was used as a negative electrode material, but an intermetallic compound cannot be easily produced from a molten metal of NiSi ₂ by an industrial metallurgical process. As is clear from the binary phase diagram of Ni-Co,
At the position where Ni contains 66.7 at% Si, there is an intermetallic compound of αNiSi ₂ , but as the NiSi ₂ melt cools, 1
At a temperature slightly higher than 100 ° C., Si precipitates and grows. NiSi whose molten metal is cooled to the eutectic point of 966 ℃
NiSi ₂ is formed. However, the amount of NiSi ₂ intermetallic compounds is very small. As described above, NiSi ₂ cannot easily produce an intermetallic compound from a molten metal, and therefore has low industrial utility value.

An object of the present invention is to provide a lithium secondary battery having a metal silicide having a large discharge capacity as a negative electrode material.

[0012]

SUMMARY OF THE INVENTION The present invention has been made in the course of solving the conventional technical problems by using a silicide with a metal as a negative electrode material. That is, Ni, F
This is because the silicide of e, Co, and Mn is found to have a remarkable improvement in charge / discharge capacity by being crystalline or partially containing amorphous. Further, by partially depositing metals such as Sn and V, further improvement can be obtained. The negative electrode material according to the present invention is an intermetallic silicide as described above, the composition of _{M 100-x Si x (x} ≧
M is at least one element selected from Ni, Fe, Co, and Mn. When the amount of silicon exceeds 50 at%, it becomes easy to precipitate silicon crystals. On the other hand, if it is 90 at% or more, it becomes difficult to industrially produce the same, so the range of 55 to 85 at% is preferable.

Furthermore, _{xy 100-M} was added an additive to the negative electrode material described above _{B y Si x (x ≧ 50at} %, 0 <y <10at%)
The negative electrode material represented by the general formula can improve the discharge capacity.
In this formula, M is at least one element selected from Ni, Fe, Co, and Mn, and B is Sn, V, Cu, Ag, A
l is a silicide containing at least one selected from l. B
The elements Sn, V, Cu, Ag, and Al represented by have the effect of increasing the discharge capacity. The limit is 10 at%. In particular, Sn and V
Has a higher discharge capacity than other additive elements, but the preferred range is 3 to 8 at%. The element represented by B functions to promote the alloying action of silicon and lithium ions during charging and the decomposition action during discharging. In particular, Sn and V precipitate between the dendritic silicon crystal and the metal silicide, and have the effect of increasing the discharge capacity over other elements.

By making the above-mentioned metal silicide into fine crystals, the contact area between lithium and silicon can be increased, so that the chemical reaction can proceed efficiently. This contributes to the improvement of charging and discharging,
This leads to higher capacity. In order to obtain fine silicon crystals, for example, there is a method of quenching molten metal. When a metal silicide is produced, if it is produced by a general dissolution method, the crystal grows in the cooling process, so that the silicon crystal becomes large. However, in the molten metal quenching method, a mixture of microcrystals and amorphous can be obtained depending on production conditions.
Not all need to be microcrystals, as long as they have microcrystals to the extent that the effects of the present invention can be achieved.

Further, by forming the silicon crystal into a dendritic shape, the effect of increasing the contact area with lithium can be obtained.
The charge / discharge capacity can be increased. Compared to a case where a carbon material as a conventional material is used as a negative electrode, a lithium secondary battery having a larger charge / discharge capacity can be provided even with batteries of the same size. Ni, Fe, Co, and Mn added to silicon form an amorphous alloy with silicon and are a matrix that retains fine dendritic silicon crystals.At the same time, they serve as electrodes for transferring electrons between silicon atoms and electrons. There is action. Hereinafter, specific examples of the present invention will be described in detail.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, a method for preparing a sample will be described below. The composition metal is weighed to a predetermined molar ratio, and is subjected to high frequency melting in the atmosphere. A sample flake was prepared by a single roll method (a method of pouring the molten metal on a copper roll having a peripheral speed of 30 m / s and rapidly cooling at a speed of 10 ⁴ K / sec or more). There are various quenching methods, and the cooling rate in the method of putting into a water tank is 10 ² K / se.
c, about 10 4 ^-5 K / sec by atomizing method or the like which sprays a metal material dissolved with nitrogen gas or water.
In order to obtain the fine Si crystal of the present invention, the rapid cooling method of putting into a water tank results in the growth of silicon crystals, and the cooling rate is insufficient. Therefore, cooling by an atomizing method, a single-roll method or the like is suitable, but the method of the present invention can sufficiently obtain the effects of the present invention as long as fine crystals can be obtained.

The flakes obtained by the single roll method are roughly pulverized by a disk mill, passed through a sieve having an opening of 100 μm, and finely pulverized by a jet mill to make the pulverized particle diameter constant and then used as a negative electrode material. . If the particle size is too large, the average particle size of the pulverized powder is not more than 30 μm in consideration of the fact that the contact surface area with lithium ions cannot be more than a predetermined value, and in consideration of the filling rate of the conductive binder in the negative electrode forming step. desirable. However, if the particle size is too small, a large amount of binder is required, and the resistance of the electrode part becomes high.
The degree is appropriate. Graphite (K
A conductive binder was prepared by mixing 21 wt% of S-15) and 7 wt% of polyvinylidene fluoride (PDVF) as a binder. 7% by weight of polyvinylidene fluoride is obtained in consideration of workability in pressure bonding. The test powder was prepared by kneading the sample powder and the conductive binder and pressing them together (press pressure: 1 t / cm ² ).

As shown in FIG. 1, the charge / discharge test apparatus comprises a device capable of charging / discharging with a constant current power supply, an electrolyte bath, a test electrode, a reference electrode (lithium foil), a positive electrode (lithium foil), and the like. You. Since this charge / discharge test apparatus is used for testing the characteristics of the negative electrode, the positive electrode 3 uses lithium and the test electrode 1
The charge and discharge characteristics were measured while replacing. As the electrolytic solution 12, a solution obtained by dissolving 1 mol / liter of LiPF ₆ in an organic solvent obtained by mixing ethylene carbonate (EC) and methyl ethyl carbonate (MEC) at a ratio of 1: 2 was used. Flip switch 7 to the charging side, and set the current density
The power supply 23 was controlled so as to be 0.5 mA / cm ² , and charging was performed until the potential of the opposing reference electrode 2 dropped to 10 mV. To measure the discharge capacity, the switch 7 is switched to discharge until the potential of the test electrode 1 rises to 0.8 V with respect to the reference electrode 2, and the quantity of electricity is first determined from the current and time. The value obtained by dividing the quantity of electricity by the weight of the sample was calculated as the discharge capacity.

[0019] According to creating method described above, M _100-x Si _x to negative electrode material: to produce a (x at%), and a discharge capacity was measured using the same. The results are shown in FIG. 2 as a relationship between the silicon content of the intermetallic silicon compound and the discharge capacity.
As shown in the figure, when the silicon content exceeds 50 at%, the discharge capacity can be rapidly increased and tends to saturate from around 85 at%. It can be seen from FIG. 2 that the characteristics exceeding 500 mA-h / cc can be obtained by containing silicon at 50 at% or more, and the discharge capacity can be greatly improved at 70 at% or more as compared with conventional carbon.

[0020] Then, M _100-x Si x and M _100-xy B
In _y Si _x, to prepare a negative electrode material for those with different x, a y variously to obtain the discharge capacity by using the same. Where M = Ni
The negative electrode materials having compositions of x = 56 at%, 67, 71, 73, and 85 are referred to as Examples 1, 2, 3, 4, and 5, respectively, and those having a composition of x = 45 at% are comparative examples. It was set to 1. As M, Example 6 containing Ni and Mn was used. For those containing an additive element B is, x = 71 at%, y = 5 at% of Ni ₂₄ Cu and B in B ₅ Si _71,
Examples 7 to 11 were Sn, V, Ag, and Al. For comparison with the current material, the test electrode 1 was made of carbon and evaluated by the same method. An alloy having the same composition as that of Example 2 but obtained by gradually cooling the molten metal from 1400 ° C. to normal temperature was used as Comparative Example 3. Table 1 summarizes the results of these evaluations.

[0021]

[Table 1]

[0022] As can be seen from Table 1, Ni, shown as Example (Mn) _100-x Si x and Ni _100-xy B _y Si
_Each of _x has an extremely large discharge capacity as compared with the conventional material (comparative example). In Ni _100-x Si _x, in accordance with x becomes 50 or more, more Then, the discharge capacity is increased.
A sample to which Sn and V are added as the additional element B has a larger discharge capacity than a sample to which Sn and V are not added.

The composition shown in Comparative Example 3 is the same as that of Example 2, except that the molten alloy is gradually cooled from 1400 ° C. to normal temperature to obtain an alloy. When the discharge capacity was measured using this alloy as a negative electrode material, the discharge capacity was 125 mAh / ml, which was extremely small as compared with that of the present invention. The X-ray diffraction pattern of the alloy of Example 2 is shown in FIG. 3, the microstructure photograph of the alloy at a magnification of 50 is shown in FIG. 4, and the X-ray diffraction pattern of Comparative Example 3 is shown in FIG. FIG. 6 shows a microstructure photograph taken at a magnification of ×. In the case of Example 2, only the silicon peak clearly appears, but in the case of Comparative Example 3, the silicon peak and the NiSi peak clearly appear, and in addition, those likely to be NiSi ₂ peaks appear. ing. The strong peak of silicon in Comparative Example 3 corresponds to the precipitation of large silicon crystals as shown in the microstructure photograph (see FIG. 6). In the case where fine silicon crystals are precipitated as in Example 2,
The discharge capacity is large.

Next, in order to see the effect of adding Sn, Table 1 was used.
10 at Sn based on the composition of Ni ₂₉ Si _{71 in} Example 3 of
% Was determined. FIG. 7 shows the result. As is clear from this, when the Sn content is in the range of 3 to 8 at%, it is greatly improved as compared with the case where no Sn is added. In particular, the maximum value is shown around 5 at%. Although the case of Sn has been described above, V also exhibits the same effect.

FIG. 8 is a micrograph of Example 8.
In this microstructure photograph, small white spots are formed in the gaps between the dendritic silicon crystals by the precipitation of Sn, which indicates that they are distributed almost uniformly in the metal silicide. When an alloy in which fine dendritic silicon crystals are present and Sn is thus precipitated is used as a negative electrode material, a lithium battery having an extremely large capacity can be obtained. It is presumed that fine dendritic silicon crystals form an alloy with lithium ions and reversibly occlude and release lithium ions. According to the microscope observation, it was found that the proportion of fine dendritic silicon crystals generated by the quenching process was higher in the example having a larger capacity.

In each of the above embodiments, M is Ni
However, as shown in Example 6, a part thereof may be substituted with Mn. Also, instead of Ni, F
One or more of e, Co, and Mn can be used in combination.

[0027]

According to the present invention, the charge / discharge capacity can be greatly improved as compared with carbon conventionally used for a negative electrode, and a silicide formed with a non-flammable metal is used. Performance and reliability can be improved.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a charge / discharge test apparatus.

FIG. 2 is a graph showing discharge capacity characteristics when the amount of Si is changed in the negative electrode material used in the present invention.

FIG. 3 shows a negative electrode material of Example 2 used in the present invention (quenched Ni
₃₃ Si ₆₇ is an X-ray diffraction pattern of the alloy).

FIG. 4 shows a negative electrode material of Example 2 used in the present invention (quenched Ni
_{33 is a} microstructure photograph of ( _33Si67 alloy).

FIG. 5 is an X-ray diffraction diagram of a negative electrode material (slow-cooled Ni ₃₃ Si ₆₇ alloy) of Comparative Example 3.

FIG. 6 is a micrograph of a negative electrode material (slow-cooled Ni ₃₃ Si ₆₇ alloy) of Comparative Example 3.

FIG. 7 is a graph showing discharge capacity characteristics when the amount of Sn added to the negative electrode material used in the present invention is increased.

FIG. 8 shows Example 8 (quenched Ni ₂₄ Sn ₅ Si) used in the present invention.
₃₁ is a photomicrograph of a negative electrode material of Example _{No. 71} alloy).

[Explanation of symbols]

1 test electrode, 2 reference electrode (Li), 3 positive electrode, 11
Tank, 12 electrolyte, 21 power supply, 22 discharge ammeter, 2
3 power supply, 24 ammeters for charging

(72) Inventor Akira Kawakami 1-1-88 Ushitora, Ibaraki City, Osaka Prefecture Inside Hitachi Maxell Co., Ltd. (72) Naoki Shinoda 1-188 Ushitora, Ibaraki City, Osaka Prefecture Hitachi Maxell Stock In company

Claims (7)

[Claims] 1. A lithium secondary battery comprising a porous separator between a positive electrode and a negative electrode, wherein the positive electrode is provided with a lithium oxide, and is filled with a non-aqueous electrolyte. wood is _{M 100-x Si x (x} ≧ 50at
%), Wherein M is at least one element selected from the group consisting of Ni, Fe, Co, and Mn. 2. The method according to claim 1, wherein the negative electrode material is 55 <
A lithium secondary battery characterized by x <85 at%. 3. A lithium secondary battery comprising a porous separator between a positive electrode and a negative electrode, wherein the positive electrode is provided with a lithium oxide, and is filled with a non-aqueous electrolyte. Material is _{M 100-xy B y Si x} (x ≧ 50
at%, 0 <y <10 at%), and M is N
A lithium secondary battery, wherein at least one element selected from i, Fe, Co, and Mn, and B is at least one element selected from Sn, V, Cu, Ag, and Al. 4. The lithium secondary battery according to claim 3, wherein the negative electrode material contains at least 3 to 8 at% of Sn or V of B. 5. The lithium secondary battery according to claim 1, wherein the negative electrode material is entirely crystalline or partially amorphous. 6. The lithium secondary battery according to claim 5, wherein the crystalline material of the negative electrode material has dendritic Si crystals. 7. The lithium secondary battery according to claim 4, wherein B is partially deposited on the negative electrode material.