JP3119945B2 - 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 having an improved charge / discharge cycle by doping lithium into a carbonaceous material constituting a negative electrode without adding a doping step.

[0002]

2. Description of the Related Art A negative electrode made of a carbonaceous material for a lithium secondary battery has attracted attention because it exhibits little durability during charge / discharge cycles and exhibits excellent durability. This is because the carbonaceous material is capable of reversibly inserting and extracting lithium at a low potential, and utilizes the reversible formation of an intercalation compound between lithium and the carbonaceous material. It is.

[0003] For example, if a battery is composed of a positive electrode containing a sufficient amount of lithium, a carbonaceous material, and a non-aqueous electrolyte through a separator, the battery is assembled in a discharged state. For this reason, this type of battery does not enter a dischargeable state unless charged after assembly. When the battery is charged in the first cycle, lithium in the positive electrode is electrochemically doped between layers of the negative carbonaceous material. When the discharge is performed, the doped lithium is dedoped and returns to the positive electrode again. However, in this type of carbonaceous material, although the degree varies depending on the type of the electrolytic solution, the undoped amount is not 100% with respect to the lithium doped amount in the first cycle. It is deactivated and remains in the carbonaceous material and no longer participates in subsequent cycles.

On the other hand, the charge / discharge reaction is carried out by moving lithium ions from the positive electrode side to the negative electrode side and from the negative electrode side to the positive electrode side, so that the amount of movable lithium is the charge / discharge capacity of the battery. However, as described above, the movable amount at the time of dedoping in the first cycle decreases, so that charging and discharging are repeated with the capacity reduced in all subsequent cycles.

Conventionally, in order to solve such a problem, a positive electrode material containing a lithium amount corresponding to a capacity loss generated in a first cycle in a negative electrode is supplemented. Before assembling the battery, a step of doping lithium into the carbonaceous material is provided in advance. Such a method is adopted.

[0006] However, the above method has a disadvantage that the volume and the weight energy density decrease because the limited internal space of the battery is occupied by the increased amount of the cathode material.

In the method of (1), the battery manufacturing process includes
It is necessary to provide a step of doping lithium into the carbonaceous material. Specific examples of these steps include a method of contacting gas phase lithium with a carbonaceous material by heating,
After mixing the carbonaceous powder and lithium metal in an inert gas or dehumidified air atmosphere, heat or pressurize, or in an organic electrolyte containing a lithium salt, place the counter electrode on the carbonaceous material electrode with lithium metal. Examples of such methods include an external short circuit and a method of performing electrolysis. However, all of them involve complicated processing, and have a drawback that costs and man-hours for installing mass-production equipment and increase in the manufacturing unit price accompanying these increases.

Therefore, as a means for solving the above-mentioned problems, as a negative electrode carbonaceous material, metallic lithium is short-circuited, placed in a battery case, housed in a battery case, and a non-aqueous electrolyte is poured into the case. A method has been proposed in which an electrochemical reaction is caused between the metallic lithium and the negative electrode by a liquid.

FIGS. 4 to 7 show application examples of this method in the case of an electrode wound type (spiral type) lithium secondary battery as an example.
Shown in These spiral type lithium secondary batteries have basically the same structure as the conventional one. That is, a winding element is formed by spirally winding the separator 3 between the positive electrode 1 and the negative electrode 2 to form a winding element, and a positive electrode lead plate 4 connected to the positive electrode 1 side is provided above the positive electrode 1 and a negative electrode 2 The negative electrode lead plate 5 connected to the side protrudes. The negative electrode lead plate 5 is connected by spot welding to the center of the inner bottom surface of the bottomed cylindrical case 6 via an insulating plate 6a, and the positive electrode lead plate 4 is spot welded to the bottom of the positive electrode terminal plate 7 with a safety valve. I have. After the non-aqueous electrolyte is injected into the case 6, the positive electrode terminal plate 7 is connected to the case 6 via a sealing gasket 8.
To complete the spiral-type lithium secondary battery by crimping.

In any of the application examples of FIGS. 4 to 7,
A lithium metal 9 is disposed in a short-circuit state with the negative electrode 2.
By applying these methods, when the non-aqueous electrolyte is injected into the battery case, the lithium metal 9 and the negative electrode 2 are electrochemically short-circuited, and the carbonaceous material forming the negative electrode 2 contains lithium. Is doped.

[0011]

However, it has been found that the following problems occur when the above method is applied. When the nonaqueous electrolyte is injected into the case, gas is generated on the surface of the lithium metal accommodated in the initial stage of the injection, and the electrolyte is deteriorated, and a part of the lithium metal is discolored and falls off. It takes about 6 to 7 days for all the lithium metal contained to dissolve and be doped into the negative electrode carbonaceous material.

The cause of the problem is that the contact area of the lithium metal with the electrolyte solution is extremely smaller than the contact area of the negative electrode of the carbonaceous material with the decomposition solution. This is because the dissolution rate of lithium metal cannot catch up with the dissolution rate. These processes will be described in detail by taking a case where the negative electrode carbonaceous material is pitch coke as an example. FIG. 8 shows that pitch coke and lithium metal were mixed with a non-aqueous electrolyte (1M LiClO ₄ / PC, DME).
(1: 1)) shows the change over time of the potential of pitch coke when external short-circuiting occurs. Since a voltage of about 3 V is generated between the pitch coke and the lithium metal at the beginning of the injection, that is, before the pitch coke is doped with lithium (part A in FIG. 8), an external short circuit occurs. It is considered that an oxidative decomposition reaction of the electrolytic solution occurred on the lithium metal simultaneously with the progress of the electrochemical reaction, and gas was generated. However, when the pitch coke absorbs a little lithium, its potential suddenly shifts to low (part B in FIG. 8), so that the voltage generated between the pitch coke and the lithium metal decreases, and the decomposition reaction Will end only if it happens instantaneously. In order to avoid this problem, it is necessary to appropriately control the injection rate so as to gradually increase the contact area between the negative electrode and the electrolyte, which is time-consuming in the manufacturing process, which is a problem.

The cause of the problem is that the potential at which the negative electrode carbonaceous material stores and releases lithium becomes very close to the lithium metal potential according to the amount of lithium stored in the carbonaceous material. In other words, the driving force that causes the short-circuit reaction between the contained lithium metal and the negative electrode carbonaceous material is the difference between the potential at which the negative electrode carbonaceous material absorbs and releases lithium and the lithium metal potential. When the potential for absorbing and releasing lithium approaches the lithium metal potential, the reaction rate of this short-circuiting reaction decreases with time (FIG. 8).
C), there was a disadvantage that the manufactured battery could not be used immediately.

The present invention solves the above problems.
The purpose is to reduce the energy density and add a doping process without lithium, the lithium is quickly doped into the carbonaceous material constituting the negative electrode active material after battery assembly, and the battery can be used immediately after battery assembly. It is an object of the present invention to provide a lithium secondary battery.

[0015]

In order to achieve the above object, according to the present invention, a positive electrode containing lithium and a negative electrode made of a carbonaceous material are arranged via a separator, and metallic lithium is short-circuited to the negative electrode. A lithium secondary battery having a structure in which the electrochemical reaction is caused by short circuit between the metallic lithium and the negative electrode by disposing the non-aqueous electrolytic solution into the case and disposing the non-aqueous electrolytic solution into the case. In the non-aqueous electrolyte, the general formula Li [Co (olefin)
(PR ₃ ) ₃ ] (where olefin is ethylene or propylene, and R is a methyl group or an ethyl group), or a general formula [Co (olefin) (P
R ₃ ) ₃ ] is 0.01 mol /
It is added in advance at a concentration of 1 mol or more and 0.1 mol / l or less.

The cobalt complex is used for intercalating an alkali metal such as potassium, sodium, lithium or the like between crystallite layers of a carbonaceous material in a solvent, and has found various properties. .

Taking lithium as an example, powdered lithium metal and graphite are represented by the general formula [Co (olefin)
(PR ₃ ) ₃ ] (where olefin is ethylene or propylene and R is a methyl group or an ethyl group), the cobalt complex acts as shown in FIG. It is known to curl. FIG. 1 first shows [Co (ol
efin) (PR ₃ ) ₃ ] contacts and reacts with lithium metal to produce Li [Co (olefin) (PR ₃ ) ₃ ], and Li
When [Co (olefin) (PR ₃ ) ₃ ] intercalates lithium into graphite, [Co (olefin)]
(PR ₃ ) ₃ ].

At this time, Li [Co (olefin) in FIG.
(PR ₃ ) ₃ ] intercalates lithium into graphite by Li [Co (olefin) (P
R ₃ ) ₃ ] is more precious than the lithium metal potential, but has a lower potential than the lithium-graphite-intercalation compound.

As will be described later, if the amount of the cobalt complex of the present invention is too small, its effect is small, and if it is too large, the internal resistance of the battery increases and the continuous discharge time until the discharge end voltage is reached is shortened. Therefore, the addition amount is limited to the above range.

In this case, as the positive electrode material, any material may be used as long as it is used for 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 or the general formula Li
MO ₂ (where M represents at least one of Co and Ni. Therefore, for example, LiCoO ₂ or LiCo
_0.8 Ni _0.2 O ₂ ) or an intercalation compound containing lithium.

The non-aqueous electrolyte is prepared by appropriately combining an organic solvent and an electrolyte, and any of these organic solvents and electrolytes can be used as long as they are used for this type of battery. For example, examples of the organic solvent include propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolan, 4-methyl-1,3 dioxolan, diethyl ether, sulfolane and the like. It is. As the electrolyte, Li
ClO ₄ , LiAsF ₆ , LiBF ₄ , LiPF ₆ , L
iCF ₃ SO ₃ , LiCl and the like.

[0022]

According to the present invention having the above-described structure, the injection of the non-aqueous electrolyte dissolves the lithium metal disposed in a short-circuit state with the negative electrode carbonaceous material, thereby doping the carbonaceous material with lithium. , A lithium-cobalt complex Li [Co (olefin) (PR ₃ ) ₃ ] added in advance. Therefore, the electrode potential of the carbonaceous material shifts to a lower speed more quickly, so that the oxidative decomposition reaction of the non-aqueous electrolyte is hardly caused. Li [Co (olefi
n) (PR ₃ ) ₃ ] is obtained by doping the carbonaceous material with lithium and changing to [Co (olefin) (PR ₃ ) ₃ ] (FIG. 1).
A part in the middle). [Co (olefin) (PR ₃ ) ₃ ] reacts with the negative electrode carbonaceous material and the lithium metal arranged in a short-circuit state to form Li [Co (olefin) (PR ₃ ) ₃ ] (b in FIG. 1). Department). Li [Co (olefin) (P
[R ₃ ) ₃ ] is obtained by doping lithium into a carbonaceous material, and changes to [Co (olefin) (PR ₃ ) ₃ ] again (part a in FIG. 1). As described above, (a) → (b) → shown in FIG.
By repeating the circulation of (a), doping of the carbonaceous material with lithium is promoted, and the battery can be used immediately after assembly. Cobalt complex [Co (ol)
efin) (PR ₃ ) ₃ ] alone (a) →
By repeating the cycle of (b) → (a), doping of the carbonaceous material with lithium is promoted, and a part of the effects of the present invention can be sufficiently obtained.

[0023]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. FIG. 2 is a side sectional view showing the structure of an AA lithium secondary battery according to one embodiment of the present invention. Basically, this lithium secondary battery is spirally wound with a separator 3 made of a polypropylene (PP) porous film sandwiched between a positive electrode 1 and a negative electrode 2 as in the prior art to form a winding element. It is formed. With the positive electrode lead plate 4 connected to the positive electrode 1 protruding above the winding element and the negative electrode lead plate 5 connected to the negative electrode 2 protruding below the winding element, the negative electrode lead plate 5 The case 6 is connected to the center of the inner bottom surface of the bottomed cylindrical case 6 by spot welding, and the positive electrode lead plate 4 is spot welded to the bottom of the positive electrode terminal plate 7 with a safety valve. Thereafter, a non-aqueous electrolytic solution is injected into the case 6, the positive electrode terminal plate 7 is fitted into the opening of the case 6 via the sealing gasket 8, and completed by caulking.

The positive electrode 1 is made of LiCo as a positive electrode active material.
O ₂ , carbon powder as a conductive agent, and an aqueous dispersion of polytetrafluoroethylene (hereinafter abbreviated as PTFE) were mixed at a weight ratio of 100: 10: 10, and kneaded into a paste with water. The mixture is applied to both sides of a 30 μm-thick aluminum foil constituting a current collector, then dried, rolled, and cut into a predetermined size, and the mixture is orthogonal to the longitudinal direction of the band. A part of the agent was scraped off, and the positive electrode lead plate 4 was spot-welded there. In the above mixing ratio, the ratio of the aqueous dispersion of PTFE is the ratio of the solid content therein.

LiCoO ₂ , which is the positive electrode active material, comprises cobalt oxide (CoO) and lithium carbonate (Li ₂ CO ₃ ).
Were mixed at a molar ratio of 2: 1 and dried in air at 900 ° C. for 9 hours. The theoretical charge amount of the positive electrode 1 is 500 mAh.

In the negative electrode 2, a mixture obtained by kneading a carbonaceous material and an aqueous PTFE dispersion at a weight ratio of 100: 3 with water is pressed into a nickel expanded metal, dried and cut. The negative electrode lead plate 5 is formed by stripping a part of the mixture at right angles to the longitudinal direction and spot welding the negative electrode lead plate 5 thereto. In addition,
The ratio of the aqueous dispersion of PTFE is the ratio of the solid content as described above, and the weight of the carbonaceous powder of the negative electrode is 3.5 g.
It is.

In the negative electrode 2, when the counter electrode is metallic lithium and the doping amount of lithium in the first cycle is 1 mA / cm ² , 800 mAh up to a voltage between both electrodes of 0.01 V and the undoping amount is 500 mAh when performed at a constant current of a density of 1 mA / cm ² .

The non-aqueous electrolyte is lithium perchlorate (L
iClO ₄ ) dissolved in a mixed solvent of propylene carbonate and 1,2-dimethoxyethane at a ratio of 1 mol / l was used.

During the assembly process, as shown in FIG.
g of the U-shaped nickel wire 10 to which the foamed lithium metal 9 is press-bonded is short-circuited to the negative electrode 2 by spot welding together, and then the non-aqueous electrolyte is injected.

Further, the electrolyte contains Li [Co (C ₂ H ₂ ) (P (C
H _{3) 3) 3]} respectively 0mol / l, 0.001mol / l
, 0.005 mol / l, 0.01 mol / l, 0.05 mol / l
, 0.1 mol / l, 0.2 mol / l and 0.4 mol / l to complete the battery.

The cobalt complex [Co (C ₂ H ₂ )]
(P (CH ₃ ) ₃ ) ₃ ] is also 0.001 mol / l, 0.005 mol / l, 0.01
mol / l, 0.05 mol / l, 0.1 mol / l, 0.2 mol / l
, At a concentration of 0.4 mol / l to complete the battery.

<Experiment 1> After assembling these batteries, the upper limit voltage was 4.2 V, the lower limit voltage was 2.8 V, and charging and discharging were performed for 20 cycles at a constant current of 0.17 A, and the discharge capacity at the 20th cycle was measured. . In addition, a cobalt complex,
Alternatively, for a battery to which no lithium-cobalt complex was added, charge and discharge were started after leaving the battery for 140 hours after assembly.
As a result, the characteristics shown in FIG. 3 were obtained. In this graph, when the upper limit of the present invention exceeds the upper limit of 0.1 mol / l, the discharge capacity rapidly decreases, but the internal resistance of the battery increases due to the addition of an excessive amount of the cobalt complex. Suggest that. In general, the discharge capacity of a battery with a lithium-cobalt complex or a cobalt complex added is lower than that of a battery without a lithium-cobalt complex, but is within an allowable range up to about 320 mAh.

<Experiment 2> During the battery assembling process, it was confirmed whether or not gas was generated during liquid injection. Table 1 shows the results. In this table, when a lithium-cobalt complex was added, gas generation during injection, that is, decomposition of the electrolytic solution was not confirmed within the scope of the present invention.

[0034]

[Table 1] << Experiment 3 >> Immediately after assembling, these batteries were charged at a constant current of 0.17 A to 4.05 V, allowed to stand for 15 minutes, and then the voltage (a) of the batteries was measured. In addition, this battery
After standing for 0 hours, the voltage (b) was measured again, and the measured values shown in Table 2 were obtained. In this table, the addition concentration of the cobalt complex or the lithium-cobalt complex is 0.01 mol /
When it was less than l, the voltage b value became higher than the voltage a value. It is considered that this is because even after the end of the charging process, all the lithium metal arranged in a short-circuit state with the negative electrode remains undissolved and remains, and this short-circuit reaction continues.

[0035]

[Table 2] As described above, from << Experiment 1 >> to << Experiment 3 >>, the addition concentration of the lithium-cobalt complex or the cobalt complex in the non-aqueous electrolyte used in the lithium secondary battery according to the present invention was within the above range (0.01 mol / l). Above 0.1 mol / l).

In this embodiment, as the lithium-cobalt complex, Li [Co (C ₂ H ₂ ) (P (C
H ₃ ) ₃ ) ₃ ], but the general formula Li [Co (Olefi
n) (PR ₃ ) ₃ ] (where Olefin is ethylene or propylene, and R is a methyl group or an ethyl group), which can perform the above-mentioned reversible reaction in a non-aqueous electrolyte. Any can be adopted. In addition, it goes without saying that the present invention can be applied not only to a spiral type lithium secondary battery, but also to a flat type lithium secondary battery such as a coin type.

[0037]

As described in detail in the above embodiment, according to the lithium secondary battery of the present invention, the lithium metal arranged in a short-circuit state with the negative carbonaceous material by the injection of the non-aqueous electrolyte is removed. The carbonaceous material is dissolved with lithium, and lithium-
Since it is also doped from the cobalt complex Li [Co (olefin) (PR ₃ ) ₃ ], doping of the carbonaceous material with lithium is promoted, and the battery can be used immediately after assembly.
Further, a cobalt complex [Co (olefin)] is added to the non-aqueous electrolyte.
(PR ₃ ) ₃ ] is added, the circulation of (a) → (b) → (a) in FIG.
Doping of the carbonaceous material with lithium is promoted, and a part of the effects of the present invention can be sufficiently obtained.

The lithium-cobalt complex dissolved in the non-aqueous electrolyte reacts with the carbonaceous material constituting the negative electrode to form a binary intercalation compound of the lithium-carbonaceous material,
Since the amount of lithium corresponding to the capacity loss at the time of the second charge / discharge is compensated for from the electrolytic solution into the carbonaceous material as the negative electrode, a complicated lithium doping step for the carbonaceous material is not required, thereby increasing equipment costs. And a low-cost, high-quality lithium secondary battery can be provided.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a reversible reaction mode of a lithium-cobalt complex in a battery.

FIG. 2 is a cross-sectional view of a spiral-type lithium secondary battery according to an embodiment of the present invention.

FIG. 3 is a graph showing the relationship between the added concentration of a lithium-cobalt complex or a cobalt complex and the discharge capacity after 20 cycles of the battery.

FIG. 4 is a side sectional view showing an example of a conventional spiral lithium secondary battery.

FIG. 5 is a side sectional view showing another example of a conventional spiral lithium secondary battery.

FIG. 6 is a side sectional view showing still another example of a spiral lithium secondary battery according to the related art.

FIG. 7 is a side sectional view showing still another example of a spiral lithium secondary battery according to the related art.

FIG. 8 is a graph showing a change in potential of pitch coke when externally short-circuiting pitch coke and lithium metal.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Negative electrode 3 Separator 4 Positive electrode lead plate 5 Negative electrode lead plate 6 Case 7 Positive electrode terminal plate 8 Sealing gasket 9 Foamed lithium metal 10 Nickel wire

Continued on the front page (72) Inventor Hidenori Nakura 5-36-11 Shimbashi, Minato-ku, Tokyo Inside Fuji Electric Chemical Co., Ltd. (72) Inventor Takashi Suzuki 5-36-11 Shimbashi, Minato-ku, Tokyo Fuji Electric JP-A-5-258771 (JP, A) JP-A-5-47416 (JP, A) JP-A-4-242074 (JP, A) JP-A-1-206571 (JP) , A) (58) Field surveyed (Int.Cl. ⁷ , DB name) H01M 10/40

Claims (2)

(57) [Claims] A positive electrode containing lithium and a negative electrode made of a carbonaceous material are arranged via a separator,
Metal lithium is short-circuited to the negative electrode, placed in a case and accommodated in a case, and an electrochemical reaction is caused between the metal lithium and the negative electrode by injecting a non-aqueous electrolyte into the case. In a lithium secondary battery having a structure, the non-aqueous electrolytic solution has the general formula Li [Co (olefin) (PR ₃ )
₃ ] (however, olefin is ethylene or propylene, R is a methyl group or an ethyl group), and a lithium-cobalt complex represented by the formula (1) is added in a concentration of 0.01 mol / l to 0.1 mol / l in advance. Lithium secondary battery. 2. The non-aqueous electrolyte has a general formula [Co (olefi
n) (PR ₃ ) ₃ ] is 0.01 m
A lithium secondary battery, which has been previously added at a concentration of ol / l or more and 0.1 mol / l or less.