JP3416180B2 - Lithium secondary battery - Google Patents

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 which prevents harmful effects due to overcharge.

[0002]

2. Description of the Related Art For example, in a flat type lithium secondary battery such as a coin type or a button type, lithium ions move to the positive electrode side during discharging, and lithium ions move to the negative electrode side during charging. Since it has a sufficient capacity according to the drop and has good cycle characteristics, it is suitable as a power source for memory backup of a computer.

As the carbonaceous material used as the negative electrode active material, a material exhibiting excellent durability with little deterioration during charge / discharge cycles is used. For example, pitch coke, natural graphite, or some organic high-grade material. Materials in which a molecular compound is carbonized or graphitized are used.

[0004] The negative electrode of a battery using this type of carbonaceous material is normally charged because when it is charged, lithium ions are occluded and move to the base direction and store lithium in an ionic form. Then, the negative electrode does not reach the metal potential of lithium, but the following problems occur due to total or partial overcharge.

[0005]

In the case of overall overcharging: Generally, a charger is incorporated in a memory backup power supply circuit, and when the battery is charged to a capacity set by a trickle charging method or a floating charging method. Charging stops, but due to a sudden failure of this charger, a large current may temporarily flow to the battery, or even if the battery reaches the end-of-charge voltage, the energized state may continue.

When the negative electrode potential of such an overcharged battery reaches a base state lower than the metallic lithium potential, metallic lithium is deposited on the negative electrode carbonaceous material. Since the deposition form here is dendrite-like, the electrodeposited lithium penetrates the porous separator and causes an internal short circuit, which then disables the operation of the battery and may lead to rupture and ignition. This is a safety issue.

In the case of partial overcharge: Even when the battery is used according to specifications, metallic lithium often deposits in the form of dendrite mainly in the peripheral portion of the carbonaceous material when the charge / discharge cycle is repeated.

This is explained as the edge effect. During charging, the peripheral portion of the electrode is more smoothly supplied with ions in the solution than the central portion thereof, so that the current is likely to concentrate.
Since the apparent current density is higher than that in the central portion, even if the potential of the entire negative electrode does not reach the lithium metal potential, the potential of the peripheral portion of the negative electrode is partially baser than the lithium metal potential. . Even in such a case, the same safety problem occurs as described above.

The inventors of the present invention repeated experiments on various carbonaceous materials in order to eliminate the above-mentioned adverse effects due to overcharge and partial overcharge.
It has been found that even if a carbonaceous material having a 002 of 3.75 to 4.00 (angstrom) shifts to a base direction lower than the lithium metal potential, there is no growth of metal lithium electrodeposited on the surface.

The present invention has been made based on the above findings, and its object is to use a carbonaceous material having a 002 plane spacing d002 of 3.75 to 4.00 (angstrom) as a part of the negative electrode. Provided is a lithium secondary battery capable of preventing the deposition of metallic lithium on the surface of the negative electrode due to the total or partial overcharge by using it, and preventing internal short circuits, ignitions, and bursts caused by this. Is.

[0011]

In order to achieve the above object, the present invention provides a positive electrode which is capable of lithium doping and dedoping and which contains lithium, a separator, and a negative electrode made of a carbonaceous material. It has a structure that is sequentially stacked,
In a lithium secondary battery including a non-aqueous electrolyte solution in this element, the surface of the negative electrode, which is in contact with the separator, has a surface spacing d002 of 002 planes of 3.75 to 4.0 determined by X-ray diffraction.
A layer (A) containing at least 70 wt% of a carbonaceous material in the range of 0 (angstrom) is provided, and the thickness of this layer is 20 to 50% of the total thickness of the separator and the negative electrode in the vertical direction. Occupy and said overall thickness
The portion corresponding to the remaining ratio is the surface spacing d0 of the 002 surface.
02 is 3.72 (angstrom) or less carbonaceous material
A layer (B) consisting of
(B) is pressure-bonded to the separator in the vertical direction .

[0012]

[0013]

[0014]

In the above structure, 00 by X-ray diffraction
Even if a carbonaceous material with a surface spacing d002 between the two surfaces of 3.75 to 4.00 (angstrom) is transferred to the base direction within 0.1 V of the lithium metal potential, the quantity of electricity is within 2000 mAh / g. It has been found by the present inventors that if so, there is no development of metallic lithium electrodeposited on the surface of the negative electrode made of a carbonaceous material. Set the lower limit of d002 to 3.
It is known that the carbonaceous material doped with lithium has an interlayer distance, that is, an interplanar spacing d002 of 002 planes of 3.72 (angstrom), which is equivalent to this interplanar spacing. This is because it is presumed that by setting the interlayer distance to be equal to or more than that, the deposited lithium is absorbed or deposited in the layer. Actually, d002 is 3.75 (angstrom)
It was confirmed that lithium metal was deposited on the carbon electrode when the above-mentioned operation was performed using a carbon material of less than 1. Also d0
The upper limit of 02 is set to 4.00 (angstrom) for the following reason. For example, when such a carbonaceous material is obtained by carbonizing a certain kind of organic polymer compound, generation of oxygen, nitrogen, hydrogen and hydrocarbon gas is not completed unless the carbonization temperature is 900 ° C. or higher. Therefore, it is difficult to obtain stable reproducibility as a carbide. For this reason, when the treatment is carried out at a carbonization temperature of 900 ° C., the value of d002 of the obtained carbide becomes 4.00 (angstrom) or less.

Further, 70% by weight of such a carbonaceous material is used.
The thickness of the above-mentioned layers is defined as 20% or more of the total thickness of the separator and the negative electrode in the vertical direction because the interplanar spacing d00 of the 002 plane by X-ray diffraction that is generally used.
This is because, when 2 is less than 20% by the combination with the carbon negative electrode capable of occluding and releasing lithium of less than 3.72 (angstrom), the absorption effect of the above-described electrodeposited lithium becomes small.

The upper limit of 50% is defined as 50%.
If the value exceeds the above, the battery capacity cannot be sufficiently secured, and it becomes difficult to obtain good characteristics as a secondary battery of this type.

The surface spacing d002 of the 002 plane by X-ray diffraction
Is 3.75 to 4.00 (angstrom), and a carbonaceous material (hereinafter referred to as carbonaceous material A) is generally a non-graphitizable resin in an inert gas or nitrogen gas atmosphere of 900 to 2
It is a fired powder of carbide obtained by heat treatment at 000 ° C.

The starting materials for obtaining the carbonaceous material A include various organic polymer compounds, for example, cellulose, Kapton film, Nomex film, β-
Cyclodextrin, polyphenylene sulfide fiber,
Examples include phenolic resin, saccharose, polyvinylidene chloride resin, etc.

In the negative electrode carbonaceous material (hereinafter referred to as carbonaceous material B) combined with the above carbonaceous material A, the interplanar spacing d002 of the 002 plane by X-ray diffraction is 3.7 (angstrom) or less, and the large Lc is large. Any carbonaceous material may be used as long as it is capable of absorbing and desorbing existing lithium, but various organic polymer compounds, carbon, etc. are exemplified as exemplified in many documents and patent publications. Carbonized and graphitized fibers, coal pitch, petroleum pitch, mesophase pitch, carbonaceous spherulites, polycyclic polynuclear aromatic polycondensates (COPNA resins), etc., and the obtained carbides are carbon black, natural graphite, man-made Although graphite and the like can be mentioned, it is desirable to select a material suitable for it in consideration of battery characteristics depending on the application.

As the positive electrode material, any material may be used as long as it is used in this type of battery, but it is particularly preferable to use a material containing a sufficient amount of lithium. For example, LiMn ₂ O ₄ and the general formula LiMO ₂ (where M represents at least one of Co and Ni. Therefore, for example, LiCoO ₂ and LiCo _0.8 Ni _0.2 O ₂
And the like) are preferable.

The non-aqueous electrolytic solution is prepared by appropriately combining an organic solvent and an electrolyte, and any of these solvents and electrolytes can be used as long as they are used in this type of battery.

As the organic solvent, propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone,
Tetrahydrofuran, 2-methyltetrahydrofuran,
1,3-dioxolane, 4-methyl-1,3-dioxolane, diethyl ether, sulfolane and the like.

As the electrolyte, LiClO ₄ , LiAs
F ₆ , LiBF ₄ , LiPF ₆ , LiCF ₃ SO ₃ , L
iCl and the like.

By combining the above positive electrode, negative electrode, electrolytic solution and separator, the lithium secondary battery according to the present invention can be obtained. In addition to the flat type in which pellets made of the carbonaceous materials A and B are stacked, a spiral type can be adopted as the battery type.

[0026]

As described above, the lithium-doped carbonaceous material has an interlayer distance, that is, 00 by X-ray diffraction.
The surface spacing d002 between the two surfaces is 3.72 (angstrom)
Is known to become.

Therefore, when lithium is doped into the carbonaceous material B having a surface spacing d002 of the 002 plane of 3.7 (angstrom) or less, the interlayer distance is expanded, and the doped lithium is strongly sandwiched between the layers. It is thought that it has become.

On the other hand, the surface spacing d002 of the 002 surface is 3.7.
When lithium is doped into the carbonaceous material A having a thickness of 5 (angstrom) or more, the interlayer distance does not change, and lithium is not strongly sandwiched between the layers as compared with the case where the carbonaceous material B is doped. It is considered that the movement of lithium in the target layer is carried out smoothly.

When these two kinds of carbonaceous materials A and B become baser than the lithium metal potential in the non-aqueous electrolyte containing a lithium salt, the carbonaceous material B causes diffusion of lithium in the layer. Lithium will be deposited immediately afterwards, while
In the carbonaceous material A, the diffusion rate of lithium in the layer is high,
When lithium is electrodeposited, it is immediately absorbed in the layer, or lithium can be electrodeposited between layers having a wider interlayer distance than in the case of the carbonaceous material B. It is considered that the lithium electrodeposition does not develop.

The carbonaceous material A does not have a layered structure like graphite and has a length of several angstroms.
It is presumed that ribbon carbon of several tens of angstroms is intricately entangled with each other to form a disordered structure. Due to such a complicated three-dimensional structure, a large number of pores are present, and the electrodeposited lithium is occluded in such pores in a state like capillary condensation, or with a plurality of carbon atoms. It is believed to form a compound, or a polymeric compound.

Therefore, by using the above carbonaceous materials A and B, it is possible to suppress the growth of metallic lithium deposited on the surface of the negative electrode, prevent internal short circuit due to dendrite-like metallic lithium deposits, and at the same time, generally Charging and discharging can be performed by the same action as that of the carbon negative electrode.

[0032]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will now be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the following examples. In order to investigate the basic properties of various carbonaceous materials prior to the examples,
Preliminary << Experiment 1 >> to << Experiment 3 >> were performed.

<Experiment 1> Cellulose, polyimide resin, phenol resin, β-cyclodextrin were each heated in a nitrogen stream at a heating rate of 20.
Set 0 ° C / hour and cooling rate to 100 ° C / hour, and
Carbonization was performed by heat treatment at 50 ° C. for 4 hours and 1000 ° C. for 2 hours. Wide-angle X-ray diffraction measurement was performed on these carbides, and the interplanar spacing d of 002 planes was measured according to the Gakshin method.
The value of 002 and the value of Lc were calculated. Table 1 shows the physical property values of the obtained carbonaceous materials A.

Further, the same operation was performed on pitch coke and natural graphite (scaly flakes produced in China, purity 99.99%), which have been conventionally used as negative electrode active materials for lithium secondary batteries, and the value of d002, Lc was obtained. Was calculated. Physical properties of these carbonaceous materials B are also shown in Table 1 at the same time.

[0035]

[Table 1] << Experiment 2 >> A constant current of 0.39 mA (corresponding to a current density of 0.5 mA / cm ² ) in the non-aqueous electrolyte for the carbonaceous materials A and B obtained in << Experiment 1 >>. The lithium was doped for 50 hours (corresponding to 1950 mAh / g). The structure of the experimental apparatus at this time is shown in FIG.

First, 4 parts by weight of polytetrafluoroethylene (hereinafter referred to as PTFE) as an adhesive was kneaded with 96 parts by weight of each of the carbonaceous materials A and B, and granulated. 15 mg of this carbonaceous material is weighed and formed into a pellet having a diameter of 10 mm, and then the pellet is pressure-bonded to a stainless steel net (diameter 14 mm) with a nickel wire, and the working electrode 2 (carbonaceous material A, carbon Material electrode composed of quality material B).

Both the counter electrode 4 and the working electrode 5 were made by fixing lithium metal to a nickel wire. Also,
As the non-aqueous electrolyte solution 6, a solution prepared by dissolving 1M of lithium perchlorate in a mixed solvent of ethylene carbonate and diethyl carbonate (volume ratio 1: 1) was used, and the solution was poured into the beaker cell 1 to prepare a working electrode 5. The change in potential was measured. The measurement results are shown in FIGS.

2 and 3, all the carbonaceous materials examined were electrolyzed at a potential lower than the lithium metal potential, but cellulose, polyimide resin, phenol resin, β
-It was confirmed that metallic lithium did not deposit on the electrodes of each carbonaceous material A of cyclodextrin. On the other hand, it was confirmed that moss-like and dendrite-like metallic lithium began to deposit on the electrode of the carbonaceous material B composed of natural graphite and pitch coke after 13 hours and 10 hours, respectively. did.

[Experiment 3] Cellulose, polyimide resin, phenol resin, β for each carbonaceous material B of pitch coke and natural graphite
-Carbonaceous material A composed of each carbide of cyclodextrin is mixed at a ratio of 50% by weight, 60% by weight, 70% by weight, 80% by weight and 100% by weight, and the carbonaceous material is prepared by the same method as << Experiment 2 >>. Material poles were made. Using this electrode, the same operation as in <Experiment 2> was performed, and a constant current of 0.5 mA / cm ² was applied in a non-aqueous electrolyte at a constant current of 50.
Doping with lithium was performed for a time. After 50 hours, the working electrode was taken out from the cell and the surface condition was confirmed. The results are shown in Table 2.

[0040]

[Table 2] In both pitch coke and natural graphite, if carbonaceous material B contains 70% by weight or more of each carbonaceous material A of cellulose, polyimide resin, phenol resin, β-cyclodextrin, electrodeposition of metallic lithium is recognized. There wasn't. Therefore, it was found that the effect of suppressing the growth of metallic lithium precipitates cannot be obtained unless at least 70% by weight or more of the carbonaceous material A according to the present invention is included.

Example 1 Using the above carbonaceous materials A and B, C as shown below was used.
An R2016 flat battery was manufactured.

First, as a binder, 96 parts by weight of a mixture of carbonaceous material A (cellulose carbide) and carbonaceous material B (pitch coke) were 0, 40, 60, 70, 80 and 100%, respectively. 4 parts by weight of PTFE were added, kneaded and granulated, and then put into a mold having a diameter of 15 mm and molded at a pressure of 3 t / cm ² , to prepare a disk-shaped negative electrode having a predetermined thickness.

The thickness of each of the disk-shaped negative electrodes was 0.47 mm and was constant, and the thickness of the pitch coke (carbonaceous material B) alone and the mixture of the carbonaceous materials A and B, which compose the same, were made. The ratio to the thickness dimension after molding is 5,1
It was changed to 0, 20, 40, 50, 60, 80, 100%.

As shown in FIG. 4, these two negative electrodes 1
In order to make 0, 12 good electrical connection, diameter 14
The stainless steel nets 14 having a diameter of 14 mm were sequentially superposed on each other, and the stainless steel nets 18 having a diameter of 14 mm provided on the inner bottom of the negative electrode terminal plate 16 were pressure-bonded at a pressure of 5 t / cm ² . Then, a separator in which a propylene non-woven fabric 20 and a microporous film 22 are superposed is arranged thereon, an electrolytic solution is poured, and a positive electrode can 28 in which a positive electrode mixture 24 is arranged via a titanium expanded metal 26 is provided as a sealing gasket. Thirty
The CR2016 flat battery having the cross-sectional shape shown in FIG.

As the positive electrode mixture 24, L of the active material was used.
Cobalt oxide (CoO) and lithium carbonate (Li ₂ CO ₃ ) were mixed in a molar ratio of 2: 1 to iCoO ₂ and heated in air at 900 ° C. for 9 hours. This Li
To 80 parts by weight of CoO ₂ , 10 parts by weight of PTFE resin and 10 parts by weight of acetylene black as a conductive agent were added, kneaded and granulated, and then put in a mold having a diameter of 15 mm, 3 t / cm
^It was molded under a pressure of ² to obtain a disk-shaped electrode having a heat of 0.48 mm. This was used by being pressure bonded to a titanium expanded metal 26 having a diameter of 14 mm provided on the inner bottom of a positive electrode can 28.

As the non-aqueous electrolyte, an ethylene carbonate-diethyl carbonate mixed solution (volume ratio 1: 1) is used.
It was used as LiClO ₄ was dissolved in 1 mol / l to.

In the battery, the compounding ratio of the carbonaceous material A (cellulose carbide) to the carbonaceous material B (pitch coke), the thickness dimension of the carbonaceous material B alone, and the thickness of the mixture of the carbonaceous materials A and B after molding. Two were made for each combination of size and ratio.

After assembling this battery, the upper cutoff voltage was set to 4.2V and the lower cutoff voltage was set to 2.8V.
A charge / discharge cycle test was performed up to 100 cycles at a constant current of 1 mA. The capacity after 100 cycles is shown in Table 3.

Next, one out of two batteries of each of these specifications which had been charged / discharged for 100 cycles was charged at 6 mA at 1 mA.
It was charged for 0 hours, and the presence or absence of an internal short circuit was confirmed. The results are also shown in Table 3.

[0050]

[Table 3] When these batteries were disassembled after the test, it was confirmed that the cause of the internal short circuit was due to the development of electrodeposited metallic lithium. Further, when a battery which was not subjected to the internal short circuit presence / absence confirmation test was also disassembled, it was found that, in the battery having the specifications indicated by X in Table 3, moss-like lithium was formed on the outer peripheral portion of the negative electrode surface where the negative electrode and the separator contacted each other. Was confirmed to have been deposited.

As is clear from Table 3, if the layer containing the carbonaceous material A included in the scope of the present invention is thick, the effect of suppressing the precipitation of metallic lithium can be sufficiently obtained. However, when the layer containing the carbonaceous material A is too thick, the battery capacity decreases,
The thickness ratio of each layer is preferably limited to the range of 20 to 50%.

[0052]

As described above in detail with reference to the embodiments, the lithium secondary battery according to the present invention has, as the constituent material of the negative electrode, the surface spacing d002 of the 002 surface by X-ray diffraction on the surface in contact with the separator. At least 7 carbonaceous materials in the range of 3.75 to 4.00 (angstroms).
By providing a layer containing 0% by weight or more, growth of electrodeposited lithium on the negative electrode is suppressed during total or partial overcharge, and dendrite-like growth of electrodeposited lithium during overcharge causes internal short circuit of the battery. It is possible to prevent fire, burst, and the like caused by the above, and to improve the safety of this type of secondary battery.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing an outline of a potential measuring device.

FIG. 2 is a graph showing changes with time of the potential of the carbonaceous material electrode A.

FIG. 3 is a graph showing the potential of the carbonaceous material electrode B and changes over time.

FIG. 4 is an exploded perspective view of a CR2016 flat battery using carbonaceous materials A and B.

FIG. 5 is a cross-sectional view showing the same assembled state.

[Explanation of symbols]

10,12 Negative electrode 14 stainless steel net 16 Negative electrode terminal plate 20,22 separator 24 Positive electrode mixture 28 Positive electrode can

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshiro Harada 5-36-11 Shimbashi, Minato-ku, Tokyo Inside Fuji Denki Kagaku Co., Ltd. (72) Hidenori Nagura 5-36-11 Shinbashi, Minato-ku, Tokyo Within Fuji Denki Kagaku Co., Ltd. (56) Reference JP-A-3-137010 (JP, A) JP-A-5-121066 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H01M 4/58 H01M 4/02 H01M 10/40

Claims (2)

(57) [Claims] 1. A structure having a structure in which a positive electrode containing lithium, which can be doped and dedoped with lithium, a separator, and a negative electrode made of a carbonaceous material are sequentially stacked,
In a lithium secondary battery including a non-aqueous electrolyte solution in this element: a surface of the negative electrode, which is in contact with the separator, has a surface spacing d002 of 002 planes of 3.75 to 4.00 by X-ray diffraction.
A layer (A) containing at least 70% by weight or more of carbonaceous material in the range of (angstrom) is provided, and the thickness of this layer occupies 20 to 50% of the total thickness of the separator and the negative electrode in the vertical direction. And the total thickness
The portion corresponding to the remaining ratio is the surface spacing d00 of the 002 surface.
2 is a carbonaceous material of 3.72 (angstrom) or less
Which is composed of a layer (B) consisting of
A lithium secondary battery characterized in that (B) is pressure-bonded to the separator in the vertical direction . 2. A pressure-bonded surface between the layer (A) and the layer (B)
The lithium secondary battery according to claim 1 , wherein a metal net is interposed in the lithium secondary battery.