JPH07302589A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PURPOSE:To eliminate lithium ion shortage in a negative electrode during charge and prevent deterioration in capacity by adding the mixture of a solid lithium ion conductor and a carbon material serving as an active material in the negative electrode. CONSTITUTION:Li3N, acetylene black, a PVDF binder, and a solvent are mixed to heat-treated meso-carbon microbeads to form a pasty material, the pasty material is applied to a negative current collector, dried, then press-molded to form a negative electrode 1. Graphite, acetylene black, PVDF, and a solvent are mixed to spinel lithium-manganese oxide to form a pasty mixture, then the pasty mixture is applied to a positive current collector, dried, then press- molded to form a positive electrode 2. A porous polypropylene separator is interposed between the positive electrode 2 and the negative electrode 1, then they are wound in a roll to form a battery element. The battery element is accommodated into a battery can 3, and an electrolyte is poured to form a battery. Since the battery contains Li3N as the solid lithium ion conductor in the negative electrode, capacity drop is decreased, and 94% of the initial capacity is maintained in 200th cycles.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improving the performance of a non-aqueous electrolyte secondary battery.

[0002]

2. Description of the Related Art As electronic devices are becoming smaller and lighter, there is a growing demand for high energy density secondary batteries as their power sources. In order to meet the demand, a non-aqueous electrolyte secondary battery, which can obtain a higher battery voltage than before, has attracted attention, and its practical application has been attempted. However, it has been found that there is a problem in the cycle life of the negative electrode, and it is quite difficult to put it into practical use. However, very recently, a non-aqueous electrolyte secondary battery has been developed that uses a carbon electrode as a negative electrode, which utilizes the inflow and outflow of lithium ions into carbon, and finally the non-aqueous electrolyte secondary battery is also in the stage of practical application. It was This battery was named by the present inventors as a lithium ion secondary battery and was named 199
It was introduced to the world for the first time in 0 years (the magazine Prog
less In Batteries & Solar
Cells, Vol. 9, 1990, p. 209), it is now recognized in the battery industry and academic societies under the name of "lithium ion secondary battery", and it is said to be the next-generation secondary battery, and its practical application is being accelerated. Typically, a lithium-containing composite oxide (for example, LiC) is used as a positive electrode material.
oO ₂ , LiMn ₂ O _4, etc.) and a carbonaceous material such as coke or graphite is used for the negative electrode. In fact, using LiCoO ₂ for the positive electrode and a special carbon material (pseudo-graphite material having a certain degree of disordered layer structure) for the negative electrode, a lithium ion secondary battery having an energy density of about 240 Wh / l is obtained. , Has already been used as a power source for mobile phones and video cameras. The energy density of the existing nickel-cadmium secondary battery is 100 to 150 Wh / l, and the energy density of the lithium-ion secondary battery is much higher than that of the existing battery in the initial stage. However, the current lithium-ion secondary battery has a large deterioration in capacity with charge / discharge cycles, and its energy density is 160 to 170 Wh / hour after 500 charge / discharge cycles.
Since it drops to 1, the characteristic of a battery having a high energy density is diminished when the charge / discharge cycle is repeated.

[0003]

DISCLOSURE OF THE INVENTION The present invention relates to improvement of cycle characteristics of a lithium ion secondary battery.

[0004]

[Means for Solving the Problem] The means for solving the problem is to incorporate an individual lithium ion conductor in an electrode by mixing it with an active material.

[0005]

[Function] A lithium-containing composite oxide (LiMn ₂ O
₄ , LiCoO ₂ , LiNiO _2, etc.) and a lithium ion secondary battery using a carbon material for the negative electrode generally has a charging efficiency (discharge amount / charge amount) of about 90% in the first charge. Indicates 99% or more after the second time. But not 100%. It is considered that the main reason why the charging efficiency does not reach 100% is that an irreversible side reaction occurs in parallel with the charging reaction as an electrode reaction. Strictly speaking, the positive electrode and the negative electrode have different rates of side reactions, so that the positive electrode and the negative electrode have different charging efficiencies, and the charging efficiency of either the positive electrode or the negative electrode appears as the charging efficiency of the battery. Then, the difference between the charge and discharge efficiencies of the positive electrode and the negative electrode is integrated every time charge and discharge are repeated, resulting in a decrease in battery capacity. In charging the lithium-ion secondary battery, lithium ions corresponding to the amount of charged electricity are dedoped from the lithium-containing composite oxide in the positive electrode and are doped in carbon in the negative electrode. That is, with respect to the charge amount of 96500 coulombs, the ideal charge reaction is that 1 gram equivalent of lithium ions is extracted from the positive electrode active material and enters the carbon. In the lithium ion secondary battery using a carbon material capable of reversibly doping and dedoping lithium as a main active material in the negative electrode, an organic electrolytic solution is used as the electrolytic solution. The organic electrolytic solution is a lithium salt (for example, Li
PF ₆ , LiBF _4, etc.) are dissolved, and the electric conduction in the electrolytic solution is cation (Li ⁺ ) and anion (PF
₆ ⁻ , BF ₄ ^−, etc.).
Therefore, the transport number of lithium ion is not 1. Lithium ions doped into carbon during charging move from the positive electrode to the negative electrode through the electrolytic solution. However, since the transport number of lithium ions is not 1, the negative electrode lacks lithium ions. On the contrary, in the discharge, lithium ions are insufficient in the positive electrode. In particular, charging due to a shortage of lithium ions causes irreversible side reactions other than the doping reaction of lithium ions in carbon in the negative electrode, which causes capacity deterioration. In the present invention, by adding a solid lithium ion conductor mixed with the active material carbon to at least the negative electrode, the lack of lithium ions in the negative electrode has been eliminated, and the cause of capacity deterioration has been successfully eliminated. That is, unlike the organic electrolyte solution in which a lithium salt is dissolved in an organic solvent, the transport number of lithium ions is almost 1 in the ionic conduction by the solid lithium ion conductor, and the lithium ions in the negative electrode are insufficient even at the end of charging. Absent. Therefore, capacity deterioration due to repeated charging and discharging is extremely reduced.

[0006]

The present invention will be described in more detail with reference to the following examples.

Example 1 The present invention will be described with reference to FIG. 1 for a specific cylindrical battery. FIG. 1 shows the overall structure of the battery of this embodiment. A battery element which is a power generation element for carrying out the present invention was prepared as follows. Mesocarbon micro beads heat treated at 2800 ° C. (d ₀₀₂ = 3.37)
Å) 84 parts by weight of 4 parts by weight of lithium nitride (Li
₃ N) is mixed well, 2 parts by weight of this acetylene black and 10 parts by weight of polyvinylidene fluoride (PVDF) as a binder are further added, and the mixture is wet mixed with N-methyl-2-pyrrolidone which is a solvent. It was made into a slurry (paste form). Then, this slurry was uniformly applied to both surfaces of a 0.01 mm-thick copper foil serving as a negative electrode current collector, dried, and then pressure-molded with a roller press to form a strip-shaped negative electrode (1).

Subsequently, the positive electrode was prepared as follows. Manganese dioxide and lithium carbonate were mixed so that the atomic ratio of Li and Mn was 1.077: 2, and the mixture was baked in air at 750 ° C. for 20 hours to spinel-type lithium manganese oxide (Li _{1. 05} Mn _1.95 O ₄ ). To 90 parts by weight of this Li _1.05 Mn _1.95 O ₄ , 4 parts by weight of graphite and 3 parts by weight of acetylene black were added and mixed well, and further 3 parts by weight of polyvinylidene fluoride as a binder and N- which was a solvent. Methyl-2-pyrrolidone is added and wet mixed to form a slurry. This slurry was uniformly applied to both sides of a 0.02 mm-thick aluminum foil which is a positive electrode current collector, dried and pressure-molded with a roller press machine to prepare a strip-shaped positive electrode (2).

The negative electrode (1) and the positive electrode (2) thus prepared
A porous polypropylene separator (3) was sandwiched between the rolls and wound up in a roll to prepare a battery element as a wound body having an average outer diameter of 15.7 mm. Next, the insulating plate (14) is placed on the bottom of the nickel-plated iron battery can (4) to house the battery element. The negative electrode lead (5) taken out from the battery element was welded to the bottom of the battery can, and ethylene carbonate (EC) and diethyl carbonate (D) were prepared by dissolving 1 mol / liter of LiPF ₆ in the battery can.
A mixed solution of EC) is injected as an electrolytic solution. After that, the insulating plate (14) is also installed on the upper part of the battery element, the gasket (15) is fitted, and the explosion-proof valve (28) is installed inside the battery as shown in FIG. The positive electrode lead (7) taken out from the battery element is welded before injecting the electrolytic solution into this explosion-proof valve. On the explosion-proof valve, a closing lid (2
9) were stacked with a doughnut-type PTC switch (16) in between, and the edges of the battery can were crimped to complete a battery (A) having an outer diameter of 16.5 mm and a height of 65 mm with the battery structure shown in FIG.

Example 2 Commercially available lithium carbonate (Li ₂ CO ₃ ) and cobalt carbonate (CoCO ₃ ) were mixed so that the atomic ratio of Li and Co was 1.03: 1, and the mixture was heated in air to 900 Calcination is performed for about 10 hours to obtain LiCoO ₂ . LiCoO ₂ after firing
Since it is obtained as a very hard mass, it is pulverized into a powder with an average particle size of 0.02 mm to obtain LiCoO ₂
Was adjusted. Li _1.05 Mn _prepared in Example 1
LioO ₂ adjusted above to 75 parts by weight of _1.95 O _4.
15 parts by weight of graphite, 4 parts by weight of graphite and 3 parts by weight of acetylene black were added and mixed well, and further 3 parts by weight of polyvinylidene fluoride as a binder and N-methyl-2-pyrrolidone as a solvent were added and wet mixed. To make a slurry. The thickness of this slurry to be a positive electrode current collector is 0.02 mm
The aluminum foil was evenly coated on both sides, dried, and then pressure-molded with a roller press to form a strip-shaped positive electrode (2b). After that, this positive electrode (2b) was combined with the negative electrode (1) prepared in Example 1, and a porous polypropylene separator (3) was sandwiched between the positive electrode and the negative electrode in the same manner as in Example 1 to form a roll shape. Then, a battery element having an average outer diameter of 15.7 mm was wound up, and the battery (B) was manufactured in the same manner as in Example 1 after that.

Example 3 To 90 parts by weight of the powdery LiCoO ₂ prepared in Example 2, 4 parts by weight of graphite and 3 parts by weight of acetylene black were added and mixed well, and further 3 parts by weight of polyvinylidene fluoride as a binder. Wet mix with solvent N-methyl-2-pyrrolidone to form a slurry. This slurry is uniformly applied to both sides of a 0.02 mm-thick aluminum foil which will be a positive electrode current collector, dried and then pressure-molded with a roller press to form a strip-shaped positive electrode (2c). Then, the positive electrode (2c) and the negative electrode (1) prepared in Example 1 were combined,
Similar to Example 1, a porous polypropylene separator (3) was sandwiched between the positive electrode and the negative electrode and rolled up to form a battery element having an average outer diameter of 15.7 mm. A battery (C) was prepared in the same manner as above.

Comparative Example 1 A lithium ion secondary battery using a spinel type lithium manganese oxide as a positive electrode active material and a graphitic carbon material as a negative electrode active material was prepared by a conventional method. ) And battery (B). 2800 ° C
_Mesocarbon microbeads (d ₀₀₂
= 3.37Å) 86 parts by weight of acetylene black 4
Parts by weight and polyvinylidene fluoride (PVD) as a binder
F) 10 parts by weight was added and wet mixed with N-methyl-2-pyrrolidone as a solvent to form a slurry (paste form). Then, this slurry was used as a negative electrode current collector in a thickness of 0.0
A 1 mm copper foil was evenly applied on both sides, dried, and pressure-molded with a roller press to form a strip-shaped negative electrode (1d).
The lithium manganese oxide synthesized in Example 1 (Li
_To 90 parts by weight of _1.05 Mn _1.95 O ₄ ), ₄ parts by weight of graphite and 3 parts by weight of acetylene black were added and mixed well, and further polyvinylidene fluoride 3 was added as a binder.
Parts by weight and N-methyl-2-pyrrolidone as a solvent are added and wet-mixed to form a slurry. This slurry was uniformly applied on both sides of a 0.02 mm-thick aluminum foil serving as a positive electrode current collector, dried, and then pressure-molded with a roller press to form a strip-shaped positive electrode (2d). The negative electrode (1d) and the positive electrode (2d) thus produced were wound in a roll shape with a porous polypropylene separator (3) sandwiched therebetween in the same manner as in the example to obtain a battery having an average outer diameter of 15.7 mm as a wound battery. A device was prepared, and a battery (E) was prepared in the same manner as in the examples.

Comparative Example 2 Lithium cobalt oxide (LiCoO ₂ ) was used as the positive electrode active material.
And a lithium ion secondary battery using a graphite carbon material as the negative electrode active material by a conventional method and compared with the battery (C) according to the present invention. The same strip-shaped negative electrode (1d) as that prepared in Comparative Example 1 and the strip-shaped positive electrode (2c) prepared in Example 3 were combined, and a porous polypropylene separator was provided between the positive and negative electrodes as in Example 1. (3)
It is wound up in a roll shape with the sandwiched between and has an average outer diameter of 15.7 mm.
A battery element was prepared, and the battery (F) was prepared in the same manner as in Example 1 after that.

Charge / Discharge Cycle Test The battery (A) of Example 1, the battery (B) of Example 2 and the battery (E) of Comparative Example 1 thus prepared were all kept at room temperature for the purpose of stabilizing the inside of the battery. After the aging period of 12 hours has elapsed, the charging current is 200 mA, the charging upper limit voltage is set to 4.2 V, and the first charging is performed at room temperature for 8 hours.
The battery was charged for a period of time, and then the first discharge was performed at room temperature by a constant current discharge of 800 mA to a final voltage of 3.0V. Thereafter, each battery was subjected to a charge / discharge cyta test at room temperature. In the charge / discharge cycle test, the charging current is 400
At mA, the charge upper limit voltage is set to 4.2V, charging is performed for 4 hours, discharge is performed at a constant current discharge of 800 mA to an end voltage of 3.0V, and charge and discharge are repeated.
It was shown to. The battery (C) of Example 3 and the battery (F) of Comparative Example 2 also had a charging current of 200 mA and an upper limit of charging after a aging period of 12 hours at room temperature for the purpose of stabilizing the inside of the battery. Set the voltage to 4.1V and
As the second charge, the battery was charged at room temperature for 8 hours, and then the first discharge was carried out at room temperature by a constant current discharge of 800 mA to a final voltage of 3.0V. Then, each battery was subjected to a charge / discharge cycle test at room temperature. In the charge / discharge cycle test, the charging current was 400 mA and the charging upper limit voltage was 4.2.
V was charged for 4 hours, discharging was performed at a constant current of 800 mA to a final voltage of 3.0 V, and charging / discharging was repeated. The results are shown in FIG.

In a lithium ion secondary battery using a lithium manganese composite oxide as a positive electrode active material and a graphitic carbon material as a negative electrode active material, its charge / discharge cycle characteristics are shown in FIG. In the battery (E), the capacity is considerably reduced as the charging / discharging cycle progresses. At the time of 100 cycles, the capacity is already about 90% of the initial capacity, and at the time of 200 cycles, the capacity is 85% of the first discharge capacity. In contrast,
In the battery (A) according to the present invention in which at least the negative electrode is added and mixed with Li ₃ N, which is well known as a solid lithium ion conductor, the degree of capacity deterioration is extremely small, and
Even at the time of the 00th cycle, 94% of the first discharge capacity is maintained. Furthermore, in the battery (B) according to the present invention, the degree of capacity deterioration is further small, and 96% of the first discharge capacity is maintained even after 200 cycles. In the battery (B), this is an example in which the positive electrode also contains an ion conduction auxiliary material. That is, LiCoO ₂ is mixed in the positive electrode in addition to the lithium manganese oxide as the main active material.
CoO ₂ is Li _1-X Co in which Li is extracted to some extent.
It is known that O ₂ is a good lithium ion conductor. LiCoO ₂ added to the positive electrode also functions as an active material, and in the positive electrode, lithium ions are partially extracted and Li _1-X CoO ₂ (X is approximately 0 <X <
Since it exists in the state of 0.7), it works effectively as an ion conduction auxiliary agent, and the charge / discharge efficiency of the positive electrode main active material is favorably maintained, so that it is considered that even better cycle characteristics can be obtained.

Similarly, in the comparison between the battery (C) of the present invention and the battery (F) of the prior art when LiCoO ₂ is used as the positive electrode main active material, the charge / discharge cycle characteristics are as shown in FIG. In addition, the capacity of the conventional battery (F) decreases considerably as the charging / discharging cycle progresses. At the time of 100 cycles, the capacity becomes about 90% of the initial capacity like the battery (E), and at the time of 200 cycles, the capacity becomes 85% of the first discharge capacity. On the other hand, in the battery (C) in which at least the negative electrode was mixed with Li ₃ N known as a lithium ion conductor, the degree of deterioration was extremely small, and
The first discharge capacity of 96% is maintained even at the 0th cycle.

In the above-mentioned embodiment, as an example in which the effect of the present invention is most prominent, as the positive electrode active material, Li
_{The case where 1.05} Mn _1.95 O ₄ and LiCoO ₂ are used has been described, but in a non-aqueous electrolyte secondary battery using a carbon material capable of reversibly doping and dedoping lithium as a main active material in the negative electrode. If so, the general formula Li [Mn _{2−X AX} ] O ₄ (A is Li, Co,
Ni, Fe, etc.) spinel type lithium manganese oxide and LiN
The present invention also exhibits the same improvement effect in a battery using another lithium-containing composite oxide such as iO ₂ as the positive electrode active material. Although Li ₃ N is used as a solid lithium ion conductor to be mixed with the negative electrode active material in the above-mentioned embodiments, Li ₃ N can replace a part of the lithium with another element such as Co. Yes, Li _3-X Co _X N (0 <
When X ≦ 0.5) was mixed with the negative electrode active material, the same result as that of the above-described example was obtained. Needless to say, the similar addition effect can be expected with other solid lithium ion conductors.

[0018]

In the conventional lithium ion secondary battery using the carbon material capable of reversibly doping and dedoping lithium as the main active material in the negative electrode, the organic electrolyte is used as the electrolyte. The organic electrolytic solution is one in which a lithium salt (for example, LiPF ₆ , LiBF ₄ etc.) is dissolved in an organic solvent, and the electric conduction in the organic electrolytic solution is cation (Li ⁺ ) and anion (PF ₆ ⁻ , BF ₄ ⁻ etc.). ) Is done by the transfer of both ions. Therefore, in the charge / discharge reaction of the conventional lithium ion secondary battery, the transport number of lithium ion is not 1,
When the negative electrode is charged and when the positive electrode is discharged, a shortage of lithium ions occurs in the electrolytic solution in the electrode, and in particular, a shortage of lithium ions at the negative electrode during charging causes capacity deterioration of the battery.
In the present invention, at least in the negative electrode, by adding a solid lithium ion conductor mixed with the carbon material of the active material, eliminating the shortage of lithium ions at the time of charging in the negative electrode and eliminating the cause of capacity deterioration. You can That is, unlike the organic electrolyte solution in which a lithium salt is dissolved in an organic solvent, the transport number of lithium ions is almost 1 in the ion conduction by the solid lithium ion conductor, and the lithium ions in the negative electrode are insufficient even at the end of charging. Absent. Therefore, capacity deterioration due to repeated charging and discharging is extremely reduced. Therefore, it is possible to significantly reduce the capacity deterioration due to the charge / discharge cycle, which is a major drawback of lithium-ion secondary batteries.
A high-capacity, long-life lithium-ion secondary battery can be provided, and its industrial value is great.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing the structure of a battery in an example.

FIG. 2 is a cycle characteristic diagram of a battery using lithium manganese oxide as a positive electrode.

FIG. 3 is a cycle characteristic diagram of a battery using lithium cobalt oxide as a positive electrode.

[Explanation of symbols]

1 is a negative electrode, 2 is a positive electrode, 3 is a separator, 4 is a battery can, 5
Is a negative electrode lead, 7 is a positive electrode lead, 14 is an insulating plate, 15 is a gasket, 16 is a PTC element 28 is an explosion-proof valve, and 29 is a closed lid.

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[Procedure amendment]

[Submission date] September 20, 1994

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be amended] Title of invention

[Correction method] Change

[Correction content]

Title of invention Non-aqueous electrolyte secondary battery

Claims (2)

[Claims] 1. A battery having a positive electrode, a negative electrode, a separator and a non-aqueous electrolyte, wherein the negative electrode uses a carbon material capable of reversibly doping and de-doping lithium as a main active material. In a secondary battery, a non-aqueous electrolyte secondary battery characterized in that at least the negative electrode contains a solid lithium ion conductor mixed with a carbon material as a main active material. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the lithium ion conductor is a lithium-containing nitride.