JPH10112307A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the cycle characteristics of a nonaqueous electrolyte secondary battery. SOLUTION: A positive electrode uses manganese oxide having spinel skeletal structure as a main active material, a negative electrode is constituted with two layers of an intercalated carbon layer 31 and a metallic lithium layer 32. After charging terminal voltage reached 4.2V, negative electrode potential is always kept in the potential at which lithium metal deposits (0V vs Li<+> /Li or less), and positive electrode potential is 4.2V (vs Li<+> /Ii) or less. Manganese does not elute from the positive electrode, and the capacity deterioration attendant on charge/discharge cycles is sufficiently retarded.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art Non-aqueous electrolyte secondary batteries using lithium metal as a negative electrode have been studied for a long time with the aim of secondary batteries having a high energy density. However, in the case of a non-aqueous electrolyte secondary battery using lithium metal as a negative electrode, good cycle characteristics can be obtained when the charge / discharge depth (charge / discharge capacity × 100 ÷ total capacity of the battery) is small (20 to 30%). When the charge / discharge depth is deep (80 to 100%), the cycle characteristics are extremely poor. That is, only 20 to 30% of the total energy stored in the battery can be used to obtain a sufficient number of times of charging and discharging. This has no meaning as a high energy density battery.

In a lithium metal negative electrode, metal lithium repeatedly precipitates and dissolves when charge and discharge are repeated. The reason why cycle characteristics are poor when the charge and discharge depth is deep is due to the state of lithium metal deposition. When the lithium metal layer repeats precipitation and dissolution in a very thin range of several microns (shallow depth of discharge), the metal lithium layer is a dense precipitate, but repeats deposition and dissolution in a thick range of several tens of microns (depth of discharge) In the case of (deep), the precipitate becomes powdery, and the function as a negative electrode is gradually lost.

Therefore, a non-aqueous electrolyte secondary battery using carbon as an active material instead of lithium metal and utilizing the intercalation of lithium ions into carbon has been developed. The battery has finally entered the practical stage. This battery was successfully commercialized for the first time in the world by the present inventors, and was first introduced to the world in 1990 under the name of "lithium ion secondary battery" (Progress magazine).
s InBatteries & Solar Cell
s, Vol. 9, 1990, p209), which is now recognized as being called the "next-generation secondary battery" and is rapidly being used as a power source for mobile phones, video cameras, notebook computers, and the like.

[0005] The lithium ion secondary battery first completed by the present inventors uses coke, which is pseudo-graphite carbon, as the negative electrode active material, and uses LiCoO ₂ as the positive electrode active material. The battery reaction of such a lithium ion secondary battery is represented by a positive electrode reaction and a negative electrode reaction according to the following equations (1) and (2). In charging, the positive electrode reaction formula (1) and the negative electrode reaction formula (2) each proceed to the right, and Li ⁺ is dedoped from LiCoO ₂ at the positive electrode, and C ₆ is doped with Li ⁺ at the negative electrode (the lithium ion is carbon Intercalate to Conversely, in discharge, the expressions (1) and (2) go to the left, respectively. At the negative electrode, Li ⁺ is dedoped from Liy C ₆ , and Li ⁺
Returns to the positive electrode. Therefore, in designing such a lithium ion secondary battery, the actual amount of dedopable lithium (X) from LiCoO ₂ in the positive electrode and the practical doping of active material carbon (C ₆ ) in the negative electrode are considered. Balance the amount of lithium (Y). Since X ≦ Y in a normal design, the amount of lithium that is dedoped from the positive electrode during normal charging can be doped into the active material carbon (C ₆ ), and there is no deposition of metallic lithium on the negative electrode (however, In the case of overcharging, metal lithium is deposited on the negative electrode), and the above-mentioned disadvantages of the metal lithium negative electrode are avoided, and stable cycle characteristics are obtained. However, in this battery, since expensive cobalt is used as a main material as a positive electrode active material, a high drawback is that the battery price is high.

Therefore, in order to realize an inexpensive lithium ion secondary battery, a lithium-containing manganese oxide having a spinel structure (such as LiMn ₂ O ₄ ) has been proposed as an inexpensive cathode active material. A battery reaction of a lithium ion secondary battery using such a lithium-containing manganese oxide as a positive electrode active material is represented by a positive electrode reaction according to the following formula (3) and a negative electrode reaction according to the above formula (2). Positive _{reaction: LiMn 2 O 4 → Li 1} -zMn 2 O 4 + zLi + + e - ·· ··· (3) (0 <z ≦ 1) negative ^{_{reaction: C 6 + yLi + + e}} - → Liy C 6 ··· (2) (0 <y ≦ 1) Conventionally, even when designing such a lithium ion secondary battery, the amount of lithium that can be actually dedoped in the lithium-containing manganese oxide in the positive electrode (Z) and the actual dopable lithium amount (Y) of the active material carbon (C ₆ ) in the negative electrode
And Z ≦ Y in a normal design. Therefore, the active material carbon (C ₆ ) can be entirely doped with lithium that is undoped from the positive electrode, so that no metal lithium is deposited on the negative electrode, and the above-described disadvantages of the metal lithium negative electrode are avoided.

However, a lithium ion secondary battery using a spinel lithium-containing manganese oxide as a main positive electrode active material has poorer cycle characteristics than a lithium ion secondary battery using LiCoO2 as a positive electrode active material, and is accompanied by a charge / discharge cycle. Large capacity deterioration. It has been found that the lithium ion secondary battery using the spinel-based lithium-containing manganese oxide as the main positive electrode active material has a new problem not in the negative electrode but in the positive electrode in cycle characteristics. Batteries with a conventional design using lithium cathode as a main cathode active material with a spinel crystal structure and a carbonaceous material as the anode, a considerable amount of disassembly of the battery after repeated charge / discharge tests and analysis of the anode surface shows a considerable The presence of an amount of manganese is confirmed.
It is considered that the cause of the poor cycle characteristics is that manganese gradually elutes from the positive electrode and deposits on the negative electrode during repeated charging and discharging. It is considered that the leaching of manganese is an irreversible reaction, and the amount of manganese eluted from the positive electrode is integrated during repeated charge and discharge, and the battery capacity is reduced in proportion to the amount of manganese eluted.

Heretofore, in order to improve such cycle characteristics, a general formula LiMn ₂ -yByO ₄ (where B is Li, N) in which part of Mn of LiMn ₂ O ₄ is replaced by another element.
Lithium-containing manganese oxides having a spinel structure represented by i, Co, Fe, Cr, etc. (0 ≦ y ≦ 0.3) have been proposed, but have not been sufficiently solved.

[0009]

DISCLOSURE OF THE INVENTION The present invention is intended to provide a nonaqueous electrolyte secondary battery mainly comprising a spinel-based lithium-containing manganese oxide as a positive electrode active material, in which capacity deterioration accompanying charge / discharge cycles is sufficiently reduced. Is what you do.

[0010]

The positive electrode comprises a manganese oxide having a spinel skeleton structure as a main active material, and the negative electrode comprises two layers of a lithium intercalated carbon layer and a metallic lithium layer.

[0011]

The present inventor has reported that the lithium manganese oxide having a spinel crystal structure has a potential of 4.2 V (vs Li ⁺ / L) during charging.
i) It was found that the elution of manganese increased rapidly above the above. Note that V (vsLi ⁺ / Li) is used as a unit meaning a potential with respect to lithium metal. From this, the reason for the elution of manganese from the positive electrode in the battery described above is that even if the battery voltage is always kept at 4.2 V or less, the positive electrode potential is 4.2 V (vs Li ⁺ /
It is considered that manganese is eluted from the positive electrode in some cases because Li) or more.

Therefore, in the present invention, as long as the battery voltage is kept at 4.2 V or less, the positive electrode potential is always 4.2 V (v) in the battery.
sLi ⁺ / Li).
More specifically, in the battery according to the present invention, the negative electrode
As shown in the cross-sectional view of the electrode shown in FIG. 1, an electrode in which two layers of a carbon layer (31) intercalated with lithium and a metal lithium layer (32) are formed on a metal current collector (30). The specific method for producing the battery according to the present invention is different from the design concept of the conventional lithium ion secondary battery, and is based on the amount of dedoped lithium (Z) from the positive electrode and the active material carbon (C ₆ ) in the negative electrode. Is designed so that Y ≒ 0.7-0.8Z. Therefore, when the battery according to the present invention is charged, about 7% of lithium is contained in the positive electrode.
At the point of 0 to 80% dedoping, doping of active material carbon (C ₆ ) with lithium in the negative electrode ends, and thereafter, metallic lithium is precipitated in the negative electrode. That is, about 7 batteries
In the state charged by 0 to 80% or more, the negative electrode is composed of two layers of a lithium intercalated carbon layer (31) and a metal lithium layer (32) as shown in the electrode cross-sectional view of FIG. . By doing so, the battery voltage becomes 4.2 V
Unless otherwise described, the positive electrode potential is always 4.2 V in the battery.
(Vs Li ⁺ / Li) or less. According to FIG. 4, in the battery according to the present invention, the positive electrode potential was 4.2 V (vs Li ⁺ / L
i) Explain that there is nothing more.

FIG. 4 shows the case where the charging voltage is set to 4.2 V,
FIG. 4 shows a case where a battery according to the present invention having an outer diameter of 17.5 mm and a height of 65 mm is charged at a current of 0 mA.
4A shows a change in terminal voltage (BP) and a change in charging current (C), and FIG. 4B shows a change in positive electrode potential (CP).
And the potential change (AP) of the negative electrode. As shown in FIG. 4A, the terminal voltage increases with the charging time, and is 2 hours and 6 minutes, that is, 2.1 hours × 500 mA = 1050 mAh.
(This corresponds to about 80% of the total chargeable amount.) When the terminal voltage reaches the charging set voltage of 4.2 V, the terminal voltage is maintained at 4.2 V and charging continues thereafter. On the other hand, the charging current is 5 until the terminal voltage reaches the charging set voltage 4.2V.
00mA continues to flow, and the terminal voltage becomes the charging set voltage 4.2V.
After that, the charge decreases and almost no current flows at the end of charging, and charging is completed. The battery according to the invention shown in FIG. 4 has a practical undoped lithium amount (Z) of LiMn ₂ O _{4 in} the positive electrode and an actual dopable lithium amount of active material carbon (C ₆ ) in the negative electrode. Since the relationship with (Y) is designed so that Y ≒ 0.8Z, about 80% of the lithium in the positive electrode is de-doped, that is, the terminal voltage becomes 4.2%.
At the time when the voltage reaches V, doping of the active material carbon (C ₆ ) with lithium in the negative electrode has been completed. Therefore, the potential (AP) of the negative electrode after the terminal voltage reaches the charging set voltage 4.2V reaches the deposition potential of lithium, that is, 0 V (vs Li ⁺ / Li) or less, as shown in FIG. In the negative electrode, metallic lithium is precipitated. That is, when the battery is charged by about 80% or more, the negative electrode is composed of two layers of the lithium-intercalated carbon layer and the lithium metal layer. The terminal voltage is a difference between the positive electrode potential and the negative electrode potential (BP = CP−AP). After the terminal voltage reaches the charging set voltage 4.2V, the negative electrode potential is AP <0V (v
sLi ⁺ / Li), the positive electrode potential is CP =
BP + AP = 4.2V + AP <4.2V (vs. Li ⁺ /
Li).

As described above, the characteristics of the battery according to the present invention are about 8
When the battery is in a charged state of 0% or more, the negative electrode is always composed of a lithium metal intercalated carbon layer and a metallic lithium layer. And the positive electrode potential in the battery (vsL
i ⁺ / Li) are the same. Therefore, when the battery voltage is 4.2V
Unless otherwise described, the positive electrode potential is always 4.2 V in the battery.
(Vs Li ⁺ / Li) or less. In the battery of the present invention, lithium metal functions as a negative electrode active material for about 20% of the capacity. However, lithium metal, which covers the capacity of 20%, can be deposited and dissolved in a very thin range of several microns. The deposition of the lithium layer is dense and does not become a major obstacle in realizing practical cycle characteristics.

FIG. 5 shows the changes in terminal voltage, charging current, positive electrode potential and negative electrode potential during charging in a battery of a conventional design (battery size: outer diameter 17.5 mm, height 65 mm). Become like As shown in FIG. 5B, when the terminal voltage actually reaches the charging set voltage of 4.2 V, the charging is not completed, and the carbon of the negative electrode is still in a state where lithium can be doped. Since the negative electrode potential (AP) has not reached the lithium deposition potential (0 V), AP> 0 V. Therefore, the positive electrode potential (C
P) becomes 4.2 V (vs. Li ⁺ / Li) or more before and after the terminal voltage reaches the charging set voltage of 4.2 V, and manganese elutes from the positive electrode.

[0016]

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

Example 1 A specific battery of the present invention will be described with reference to FIGS. 6 and 7. First, 70 parts by weight of mesocarbon microbeads (d002 = 3.36 °) heat-treated at 2800 ° C. as a carbon material to be used as a negative electrode active material were dry-mixed with 20 parts by weight of pitch coke, and polyfluoride was used as a binder. Vinylidene (PVDF) was wet-mixed with N-methyl-2-pyrroline in which 10 parts by weight were dissolved to form a slurry (paste). The slurry is uniformly applied to both sides of a 0.01 mm-thick copper foil serving as a negative electrode current collector (21), and dried at a drying temperature of 110 ° C in a solvent (N-methyl-2-).
(Pyrrolidone) was completely removed to obtain a coated body in which carbon layers were formed on both surfaces of a copper foil. Further, the coated body was press-molded with a roller press, and the width was adjusted to 56 mm to prepare a strip-shaped negative electrode body. A predetermined area is cut out from the negative electrode body, the weight of the carbon layer is measured by performing weight measurement, and a test cell is prepared by combining with the lithium metal, and the amount of lithium (Y) that can be doped into the carbon layer is measured. did. The weight of the carbon layer is 21.7 mg / cm ² to 2
In the range of 2.7 mg / cm ^2, the amount of lithium (Y) that can be doped into the carbon layer is about 6.25 mAh / cm 2.
It was ² .

Next, commercially available manganese dioxide [MnO2] calcined at 400 ° C. for 19 hours and lithium carbonate [Li
2CO3] in a ratio of 1.94 mol: 0.53 mol, and calcined in air at 860 ° C. for 12 hours to produce lithium manganese oxide (LiMn2) of a spinel crystal as a positive electrode active material.
O4) was synthesized. The lithium manganese oxide after calcination was converted into a powder having an average particle size of 0.015 mm by a pulverizer.
8 parts by weight, 6 parts by weight of graphite, 6 parts by weight of polyvinylidene fluoride as a binder, N-methyl-2 as a solvent
-Slurry (paste) by wet mixing with pyrrolidone. Next, this slurry is uniformly applied to both surfaces of a 0.02 mm-thick aluminum foil serving as a positive electrode current collector (22) such that a dry coating amount is about 73 mg / cm ² .
After drying at a drying temperature of 110 ° C. until the solvent (N-methyl-2-pyrrolidone) is completely removed, the mixture was pressed and molded by a roller press, and the width was adjusted to 55 mm. Five strip-shaped positive electrode bodies on which a positive electrode active material layer containing lithium manganese oxide (LiMn2O4) was formed were prepared. A certain area is cut out from the positive electrode body, the weight of the positive electrode active material layer is measured by performing weight measurement, and a test cell is prepared in combination with lithium metal, and each positive electrode body is dedoped from the positive electrode active material layer. The possible lithium amount (Z) was measured. The weight of the positive electrode active material layer is 7
2 mg / cm ² to 74 mg / cm ² ,
The amount (Y) of undoped lithium in the positive electrode active material layer is completely proportional to the weight of the positive electrode active material layer, and is 7.8 mAh on average.
/ Cm ² .

The prepared negative electrode and positive electrode are wound up in a roll with a porous polypropylene separator interposed therebetween to form a battery element (20) having an average outer diameter of 16.9 mm. The prepared battery element (20) is housed in a battery can (1) as shown in FIG. In the present embodiment, the outer diameter (L2) of the battery can of the final completed battery is to prepare a battery with A = 17.5 mm. The battery can used here has the outer diameter (L1) of the opening. And an outer diameter (L2) of the central portion is B = 18.0 mm, and the height is 65 mm. Incidentally, the inner diameter of the battery can is 17.4 mm, and the outer diameter of the battery element is smaller than the inner diameter of the can by 0.5 mm, so that the battery element can be easily inserted into the battery can. After storing the battery element (20) in the battery can (1), the outer diameter (excluding the outer diameter near the opening) of the battery can is reduced to 17.5 mm as shown in FIG.

Thereafter, as shown in FIG.
At a position of 0.5 mm (near the opening of the battery can), the battery can is squeezed finely to form a narrow groove (2) for supporting the gasket.
Thereafter, the battery is assembled according to the steps (a) to (c) in FIG. That is, as shown in FIG. 7 (a), a gasket (3) is placed at the opening of the battery can, and the negative electrode lead and the positive electrode lead are respectively welded to the can bottom and an aluminum explosion-proof lid (4). . Thereafter, an electrolytic solution is injected, the closure lid (4) is placed inside the gasket, and the donut-shaped PTC element (5) is brought into contact with the closure lid and stacked.
Further, the positive electrode external terminal (6) is overlapped, and the outer diameter (L1) of the opening of the battery can is narrowed to be the same as the outer diameter (L2) of the central part of the battery can (FIG. 7B). Finally, the caulking machine is adjusted so that the tightening pressure does not impair the function of the PTC element, and the edge of the battery can is caulked and sealed.
5 batteries (A) having an outer diameter of 17.5 mm and a height of 65 mm having the battery structure shown in FIG.

Comparative Example 1 For comparison with the battery according to the present invention, a battery is prepared by a conventional design method. In the conventional design method, the amount of undoped lithium (Z) in the positive electrode and the active material carbon (C ₆ ) in the negative electrode
The relationship between the amount of lithium (Y) that can be doped into
And By the same procedure as in Example 1, can be doped lithium content by weight of the carbon layer is from 25.5 mg / cm ² to 26.5 mg / cm ² in the range ,, carbon layer (Y)
Produced a negative electrode body of about 6.95 mAh / cm ² .

Five positive electrode bodies were prepared in the same manner as in Example 1, except that the dry coating amount was about 65 mg / cm ² . A certain area is also cut out from the positive electrode body, the weight of the positive electrode active material layer is measured by performing weight measurement, and a test cell is prepared in combination with lithium metal, and the actual size of the positive electrode active material layer of each positive electrode body is measured. The amount (Z) of lithium that can be de-doped was measured. The weight of the positive electrode active material layer is in the range of 64 mg / cm ² to 66 mg / cm ² , and the amount (Y) of undoped lithium in the positive electrode active material layer is completely proportional to the weight of the positive electrode active material layer, 6.93 mAh / cm ² .

A battery element (20) having an average outer diameter of 16.9 mm was prepared in exactly the same manner as in Example 1 using the prepared negative electrode and positive electrode, and the battery structure shown in FIG.
Five batteries (B) having a height of 65 mm and a height of 65 mm were prepared.

Charge / discharge cycle test result Battery (A) prepared in Example 1 and Comparative Example 1
For the purpose of stabilizing the inside of the battery, the charging voltage was set to 4.1 V, and the battery (B) was charged at a current of 500 mA for 8 hours, and a final voltage of 2.5 mA was supplied at a current of 500 mA.
Discharged to V. Next, the charging voltage was set to 4.2 V and 5
The battery was charged with a current of 00 mA for 4 hours, and the discharge was repeated 300 times with a current of 500 mA up to a final voltage of 2.5 V. The results are shown in FIG.

As shown in FIG. 1, in the battery (A) according to the present invention, even at the 300th discharge, the discharge capacity is maintained at 80% or more of the initial capacity (the 10th discharge capacity). On the other hand, in the battery (B) according to the comparative example, 30
The 0th discharge capacity was reduced to 67% of the initial capacity (the 10th discharge capacity). As shown by the results, according to the present invention, the charge / discharge cycle characteristics are greatly improved.

Observation of Negative Electrode by Disassembly of Battery When the charge and discharge cycle of each of the two batteries (A) and (B) prepared in Examples and Comparative Examples was completed 10 times, the charge voltage was increased to 4.2 V. After charging was stopped for 2.5 hours at a set current of 500 mA, the battery was disassembled and the negative electrode surface was observed. The amount of electricity charged in 2.5 hours is 88% for the battery (A) and 90% for the battery (B) with respect to the discharge amount discharged from the fully charged state in the previous cycle.
Therefore, none of them are fully charged. Another battery (B) was charged at a current of 500 mA for 4 hours with the charging voltage set to 4.2 V at the end of the ten charge / discharge cycles. After disassembly, the negative electrode surface was observed. Note that a current of 500 mA
When the battery was charged for an hour, the charged amount of electricity was 100% of the amount of discharge in the previous cycle, indicating that the battery was fully charged.

The surface of the negative electrode taken out of the battery (A) was silver-white, and when a thin silver-white layer on the surface was scraped off, a golden layer appeared below it. The silver-white thin layer is a layer of metallic lithium, and the golden layer is a carbon layer in which lithium is intercalated. That is, even when only 88% of the capacity is charged, the negative electrode taken out of the battery (A) has a structure of the electrode cross section as shown in FIG. 2, and lithium is deposited on the metal current collector (30). It was confirmed that the electrode had two layers of the intercalated carbon layer (31) and the lithium metal layer (32). On the other hand, the surface of each of the negative electrodes taken out of the battery (B) was golden, and the silver-white surface layer observed in the negative electrode taken out of the battery (A) was not observed. That is, the capacity of 9
The negative electrodes taken out from both the battery (B) charged with 0% and the battery (B) charged with 100% of the capacity are shown in FIG.
The structure of the electrode cross section shown in FIG.
0) on the lithium-intercalated carbon layer (3
It was confirmed that only the electrode 1) was formed.

From the above observation results of the negative electrode, the reason why the charge / discharge cycle characteristics are greatly improved by the present invention is considered as follows. FIG. 4 shows that the charging voltage was set to 4.2 V and 5
FIG. 4A shows a case where a battery (A) according to the present invention having an outer diameter of 17.5 mm and a height of 65 mm is charged with a current of 00 mA. FIG. 4A shows a change in terminal voltage (BP) and a change in charging current. FIG. 4B shows a change in potential of the positive electrode (CP) and a change in potential of the negative electrode (AP). As shown in FIG. 4A, the terminal voltage is 2.1 hours (about 105 hours).
(0 mAh: this corresponds to about 80% of the total charged amount) When the battery is charged, the voltage reaches 4.2 V, and thereafter, the voltage is maintained at 4.2 V and the charging is continued. On the other hand, the charging current continues to flow at 500 mA until the terminal voltage reaches the charging set voltage of 4.2 V,
After the terminal voltage reaches the charging set voltage of 4.2 V, the voltage decreases and hardly flows at the end of charging, and charging is completed. Therefore, in the battery (A), when the charging time has passed 2.5 hours, the terminal voltage has already reached 4.2 V, and the potential of the negative electrode (A
P) is 0 V (vs Li ⁺ / L) as shown in FIG.
i) The following has been reached, and metallic lithium is deposited on the negative electrode. The battery according to the present invention has LiMn ₂ O ₄ in the positive electrode.
The relationship between the actual amount of lithium that can be undoped (Z) and the actual amount of lithium that can be doped with active material carbon (C ₆ ) (Y) in the negative electrode (Y) is designed so that Y ≒ 0.8Z. Therefore, when about 80% of the lithium in the positive electrode is dedoped, doping of the active material carbon (C ₆ ) with lithium in the negative electrode has already been completed. When the battery (A) is charged for 2.5 hours, the capacity of 80% or more is already charged.
At the negative electrode, metallic lithium is precipitated. Therefore, as is clear from the observation result of the negative electrode by dismantling the battery, the negative electrode taken out from the battery (A) charged for 2.5 hours has a structure of the electrode cross section as shown in FIG. The electrode has a carbon layer (31) with lithium intercalated thereon and a lithium metal layer (32) formed thereon. After the charging terminal voltage reaches 4.2 V, the negative electrode potential is always the potential at which lithium metal is deposited (0 V vs. Li).
⁺ / Li or less), and the positive electrode potential is always 4.
2 V (vs Li ⁺ / Li) or less. Therefore, it is considered that manganese does not dissolve out of the positive electrode, and the capacity deterioration due to the charge / discharge cycle becomes sufficiently small.

However, in the battery (B) according to the conventional design, the positive electrode potential is 4.2 V (vs Li ⁺
/ Li) or more. FIG. 5 shows a case where the battery (B) is charged with a charging voltage of 4.2 V and a current of 500 mA, and FIG. 5 (a) shows a change in the terminal voltage (B).
P) and a change in charging current (C), and FIG. 5B shows a change in potential of the positive electrode (CP) and a change in potential of the negative electrode (AP). As shown in FIG. 5A, the terminal voltage rises with the charging time, reaches a charging set voltage of 4.2 V in about 1.9 hours, and is maintained at 4.2 V. The charging current continues to flow at 500 mA until the terminal voltage reaches the charging set voltage of 4.2 V,
After the terminal voltage reaches the charging set voltage of 4.2 V, the voltage decreases and hardly flows at the end of charging, and charging is completed. As can be seen from the change in terminal voltage (BP) and the change in charging current (C), the battery (B) is almost similar to the battery (A) according to the present invention shown in FIG. However, at this time, the terminal voltage is the difference between the positive electrode potential and the negative electrode potential (BP = CP−
AP), when the terminal voltage reaches 4.2 V, the negative electrode potential becomes AP> 0 V (vs. Li ⁺ / Li).
Then, the positive electrode potential is CP = BP + AP = 4.2V + A
That is, P> 4.2 V (vs Li ⁺ / Li).
In the conventional design, the amount of lithium (Z) that can be dedoped in the positive electrode and the amount of lithium that can be doped into the active material carbon (C ₆ ) in the negative electrode (Y) are set to Z ≦ Y.
As shown in the figure, the terminal voltage is actually 4.2 V
At the time when the charge reaches, the charge is not completed, the carbon of the negative electrode is still in a state where lithium can be doped, and the negative electrode potential (AP) has not reached the lithium deposition potential (0 V). AP> 0V. Therefore, the positive electrode potential (CP) becomes equal to or higher than 4.2 V (vs Li ⁺ / Li) from before and after the terminal voltage reaches the charging set voltage 4.2 V, and manganese is eluted from the positive electrode. For this reason, it is considered that the capacity deteriorates with the charge / discharge cycle in proportion to the amount of manganese eluted from the positive electrode.

[0030]

The battery according to the present invention uses a spinel-based lithium-containing manganese oxide as the main positive electrode active material, so that the raw material cost is low, and the negative electrode has lithium intercalated on a metal current collector. Since the electrode has two layers of a carbon layer and a metal lithium layer, the positive electrode potential is always 4.2 V (vs Li ⁺ / Li) or less in the battery.
Therefore, manganese does not dissolve from the positive electrode, and the capacity deterioration due to the charge / discharge cycle is sufficiently small. As a result, an inexpensive lithium ion secondary battery having good cycle characteristics can be provided, and its industrial value is great.

[Brief description of the drawings]

FIG. 1 is a cycle characteristic diagram.

FIG. 2 is a sectional view of a negative electrode in the battery of the present invention.

FIG. 3 is a cross-sectional view of a negative electrode in a conventional battery.

FIG. 4 Changes in terminal voltage, current, and electrode potential during charging

FIG. 5: Changes in terminal voltage, current, and electrode potential during charging

FIG. 6 is a cross-sectional view of a battery in each assembly process.

FIG. 7 is a cross-sectional view of a battery in each assembly process.

[Explanation of symbols]

1 is a battery can, 2 is a narrow groove, 3 is a gasket 4 is a closure lid,
5 is PTC 6 is a positive electrode external terminal, 20 is an electrode element, 30 is a current collector, 3
1 is a carbon layer, 32 is a metal lithium layer, 33 is carbon particles,
34 is a binder.

Claims (1)

[Claims] 1. A secondary battery comprising a negative electrode, a positive electrode, a separator, and a non-aqueous electrolyte, wherein the positive electrode comprises 90% or less of a positive electrode active material mainly composed of a manganese oxide having a spinel skeleton structure. A non-aqueous electrolyte secondary battery in which the negative electrode is an electrode in which a two-layer of a carbon layer in which lithium is intercalated and a lithium metal layer is formed on a metal current collector even in a charged state.