JPH05144472A - Secondary battery with nonaqueous electrolyte - Google Patents

Abstract

PURPOSE:To provide a nonaqueous electrolyte secondary battery, which is embodied light and small, equipped with a high energy density, and presenting excellent resistance against over-discharging. CONSTITUTION:A metal lithium foil is in advance affixed to the carbon material of a negative electrode 2, and the lithium is allowed to permeate the carbon material through the action of the potential difference or concentration difference, and thereby the lithium for enabling charging is retained by the negative electrode carbon material. Therein the capacity of metal Li foil affixation ranges 4-40% of the saturated reversible capacity of the negative electrode carbon material. The metal lithium foil affixed to the carbon material forms a local cell together with the carbon material under existence of nonaqueous electrolytic solution, and the lithium is intercalated in the carbon material electrochemically to be retained as dischargeable lithium.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a non-aqueous electrolyte secondary battery,
In particular, it relates to improvement of over-discharge resistance of a lithium secondary battery.

In recent years, with the remarkable portable and intelligent use of cordless information / communication devices such as mobile phones and camcorders, there has been a demand for a small-sized and lightweight secondary battery having a high energy density as a power source battery for driving the cordless information / communication device. Non-aqueous electrolyte secondary batteries, especially lithium secondary batteries, are highly expected as the mainstay of next-generation batteries, and their potential market size is also very large.

[0003]

2. Description of the Related Art Conventionally, in a lithium secondary battery, a transition metal oxide or sulfide such as manganese dioxide (MnO ₂ ) or molybdenum disulfide (MoS ₂ ) is used as a negative electrode active material as a positive electrode active material. Battery systems using lithium have been proposed. However, in this battery, the lithium deposition pattern during charging is greatly affected by the composition of the non-aqueous electrolyte, charging conditions, etc., and mainly becomes needle-like or mossy, which falls off from the negative electrode or penetrates the separator. It has been said that there is a problem with safety such as contact with the positive electrode and causing internal short circuit or ignition.

Therefore, a battery system using a compound for electrochemically intercalating / deintercalating lithium in the positive and negative electrodes has been proposed. In this battery, lithium is not deposited on the electrode during charging, and it is considered that it can be expected to improve safety and at the same time has excellent rapid charging characteristics, and currently, research and development are actively carried out.

In this battery, as a positive electrode active material, a lithium-containing composite oxide of a transition metal, that is, LiMO ₂ having a layered structure or LiM ₂ O ₄ having a spinel structure (where M is a transition metal such as cobalt, Any one of manganese and nickel-iron) has been attracting attention as having high voltage and high energy density.

On the other hand, as a negative electrode material, a carbon material having a layered structure reversibly intercalates lithium /
It is regarded as a promising material for deintercalation, and the reversibility of the intercalation / deintercalation, the physical properties of the carbon material, the relationship with the structure, etc. are being actively investigated.

[0007]

As described above, by using a lithium-containing composite oxide of a transition metal for the positive electrode active material and a carbon material for the negative electrode material, respectively, the size and weight are small and the safety is high. It is considered possible to provide a non-aqueous electrolyte secondary battery having an energy density.

However, some problems still remain in this battery. One of them is improvement in over-discharge resistance.

In recent years, cordless information / communication devices are often equipped with a so-called auto power-off function in order to avoid wasting a power battery. This function is in the power-on state, when (1) the device is not driven, that is, when a certain time has passed in a so-called pause state, (2) the device is driven, and when the battery voltage reaches the set lower limit voltage, The power is automatically turned off.

When the auto power-off function is still in operation, the battery is continuously discharged by the circuit load, and eventually the battery voltage reaches 0V. Therefore, if the so-called over-discharge resistance characteristic, in which the capacity is restored by recharging even after such over-discharge, is not excellent, the practicality of the battery is extremely low.

However, in the case of a non-aqueous electrolyte secondary battery in which a lithium-containing composite oxide of a transition metal is used as the active material of the positive electrode and a carbon material is used as the negative electrode material, recharging is performed after such over-discharging. It was found that the capacity was hardly recovered and that the deterioration of capacity with the cycle was much larger than that before the overdischarge.

When a carbon material is used as the negative electrode material, the potential of the negative electrode, that is, the potential at which the carbon material intercalates / deintercalates lithium depends on the physical properties of the carbon material, particularly the degree of development of the layered structure (interlayer distance, It is about 1.5 V or less with respect to lithium, though it depends on the overlap of layers in the c-axis direction and the spread of layers in the a-axis direction.

However, when this battery was over-discharged, it was found that the potential of the negative electrode rose to about 3.2 V or more with respect to lithium and became equal to the potential of the positive electrode, and the battery voltage reached 0 V. It was

For this reason, the physical properties and structure of the carbon material change, and the reversibility of lithium intercalation / deintercalation is lost, and even if it is recharged after overdischarging, the capacity is almost restored. It is considered that the reason is that the capacity deterioration due to the cycle becomes much larger than that before the overdischarge.

The present invention solves this problem, and an object of the present invention is to improve the over-discharge resistance of a lithium secondary battery.

[0016]

The present invention is a non-aqueous electrolyte secondary battery in which a lithium-containing composite oxide of a transition metal is used for the positive electrode and a carbon material is used for the negative electrode, and the negative electrode is previously attached to the carbon material. The metal lithium foil described above is diffused into the carbon material due to the potential difference or the concentration difference, so that the carbon material of the negative electrode holds the dischargeable lithium.

Further, the sticking capacity of the metallic lithium foil is 4 with respect to the saturation reversible capacity of the carbon material used for the negative electrode material.
-40%.

Here, the saturation reversible capacity of the carbon material used as the negative electrode material was calculated by the following method. A carbon material is used for the positive electrode material and metallic lithium is used for the negative electrode material.
Constant-current charging / discharging with a current density of 0.5 mA / cm ² at 5 ° C. was repeated 5 cycles. The capacity at this time was defined as the saturated reversible capacity. The upper limit voltage during charging was 1.0V and the lower limit voltage during discharging was 0V.

In addition, the active material of the positive electrode may be of the general formula LiMO
₂ or LiM ₂ O ₄ (where M is cobalt, manganese,
(A nickel or iron) alone or a lithium-containing composite oxide in which cobalt, manganese, nickel, or iron is partially replaced by another transition metal, while the negative electrode material is a powder X-ray diffraction method. A carbon material having a lattice spacing (d ₀₀₂ ) of 0.342 nm or less is preferable.

[0020]

According to the present invention, the metallic lithium foil attached to the carbon material of the negative electrode constitutes a local battery between the metallic lithium foil and the carbon material in the presence of the non-aqueous electrolyte, and the metallic lithium is electrochemically dissolved. The carbon material is intercalated in the vicinity and is retained in the carbon material as lithium that can be discharged.

The lithium held on the carbon material does not raise the potential of the negative electrode due to discharge at the time of over-discharging. Therefore, the physical properties and structure of the carbon material do not change,
The reversibility of lithium intercalation / deintercalation into carbon material is not lost. Therefore, even in the case of a battery after over-discharging, the capacity is promptly restored by recharging, and the capacity deterioration due to the cycle does not change as compared with that before over-discharging. That is, the over-discharge resistance can be improved.

[0022]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a vertical cross-sectional view of the configuration of the cylindrical lithium secondary battery of the present invention.

The positive electrode plate 1 is made of lithium carbonate (LiCO ₃ )
And tricobalt tetroxide (Co ₃ O ₄ ) were mixed and fired at 900 ° C. in air lithium cobalt oxide (LiCo
O ₂ ) as an active material, 3% by weight of acetylene black as a conductive agent was mixed therein, and then 7% by weight of an aqueous dispersion of polytetrafluoroethylene resin was kneaded as a binder to form a paste. The agent is applied on both sides of a core material made of aluminum foil, dried and rolled. Further, the positive electrode lead plate 4 is spot-welded to the end portion thereof. The positive electrode plate has a width of 40 mm, a length of 250 mm, and a thickness of 0.170 mm.

The negative electrode plate 2 is a paste prepared by kneading 5% by weight of an aqueous dispersion of polytetrafluoroethylene resin as a binder into spherical graphite obtained by heat-treating mesophase pitch at 2800 ° C. in an argon atmosphere. The above mixture was applied to both sides of a core material made of copper foil, dried and rolled. Further, the negative electrode lead plate 5 is spot-welded to the end portion thereof. The negative electrode plate has a width of 42 mm and a length of 27.
The thickness is 0 mm and the thickness is 0.205 mm.

As a result of preliminary examination of various carbon materials having different physical properties and structures, a carbon material having a lattice spacing (d ₀₀₂ ) of 0.342 nm or less by the powder X-ray diffraction method has a high capacity. It was also found to be excellent in reversibility. Incidentally, the spherical graphite obtained by heat-treating mesophase pitch at 2800 ° C. in an argon atmosphere had a lattice plane spacing (d ₀₀₂ ) of 0.342 nm or less measured by a powder X-ray diffraction method.

As the separator 3, a polypropylene porous film cut into a wider area than the positive electrode plate and the negative electrode plate was used.

The positive electrode plate and the negative electrode plate were spirally wound as a whole with a separator interposed therebetween to form an electrode plate group.

Next, the upper and lower parts of the electrode plate group are heated by hot air to heat-shrink the separator 3. The lower insulating ring 6 is attached to the lower side of the electrode plate group, is housed in the battery case 7, and the negative electrode lead plate 5 is spot-welded to the battery case 7. Also, the upper insulating ring 8 is attached to the upper side of the electrode plate group, and the battery case 7
After grooving on the top of, the non-aqueous electrolyte is injected. The non-aqueous electrolyte is a mixture of ethylene carbonate (EC) and diethylene carbonate (DEC) in a volume ratio of 1: 1.
Lithium hexafluorophosphate (LiPF ₆ ) was dissolved at 1 mol / l. After the assembly sealing plate 9 in which a gasket was incorporated in advance and the positive electrode lead plate 4 were spot-welded, the assembly sealing plate 9 was mounted on the battery case 7 and caulked and sealed to form a battery. The size of this battery is 14mm in outer diameter and 50mm in total height.
(AAA).

Test Evaluation The over-discharge resistance of the battery configured as described above was evaluated by the following test method. First, 100 mA constant current charge / discharge at 20 ° C. was repeated 50 cycles. The upper limit voltage during charging was 4.1V and the lower limit voltage during discharging was 3.0V. After that, the battery was changed from a discharged state to an over-discharged state, and a constant resistance discharge of 1 kΩ was continued for 2 weeks. At this time, the result of observing the overdischarge behavior of the positive and negative electrodes using metallic lithium as the reference electrode is shown in FIG. Then, the constant current discharge of 100 mA was repeated again for 50 cycles. FIG. 3 shows the result of comparison between the capacity recovery characteristic and the cycle characteristic before and after over-discharging.

As is clear from FIG. 3, the capacity recovers only about 55% after over-discharging, and the capacity deterioration due to the cycle is significantly larger than that before over-discharging.

Further, as shown in FIG. 2, the potential of the negative electrode rises to 3.2 V or more with respect to lithium during over-discharging, becomes equal to the potential of the positive electrode, and the battery voltage reaches 0 V. all right.

In normal charging / discharging, the potential of the positive electrode is in the vicinity of this and there is no problem, but the potential of the negative electrode is about 0.1 V (charging / discharging).

[0033]

[Outer 1]

It is about 0.5 V (during discharge). For this reason,
Due to changes in the physical properties and structure of the carbon material, the reversibility of lithium intercalation / deintercalation is lost, and as a result, the capacity hardly recovers even after recharging after overdischarge, and the capacity associated with cycling It is considered that the deterioration is much larger than that before the overdischarge.

Example 1 A battery was constructed in the same manner as in the above case using a negative electrode plate prepared by pasting a metallic lithium foil on a carbon material in advance, and the overdischarge resistance was evaluated. As an example, FIG. 4 shows the results of observing the overdischarge behavior of the positive and negative electrodes when the sticking capacity of the metallic lithium foil was 20% of the saturated reversible capacity of the carbon material. At this time, the dimensions of the metallic lithium foil are 40 mm in width, 40 mm in length, and 0.0 in thickness.
It was set to 30 mm.

FIG. 5 shows the relationship between the sticking capacity of the metallic lithium foil and the capacity recovery characteristic as the over-discharge resistance characteristic. At this time, the size of the metallic lithium foil is 40 mm in width and 40 mm in length.
It was fixed with and the volume was adjusted by the thickness.

As is clear from FIG. 5, when the sticking capacity of the metallic lithium foil is 4% or more of the saturated reversible capacity of the carbon material, good over-discharge resistance can be obtained as compared with the conventional example. I understood.

Further, as shown in FIG. 4, it was found that the potential of the negative electrode increased to about 1.5 V with respect to lithium during overdischarge. Further, it was confirmed that similar over-discharge behavior was obtained when the attachment capacity of the metallic lithium foil was 4% or more with respect to the saturated reversible capacity of the carbon material.

This is because the metallic lithium foil attached to the negative electrode carbon material forms a local battery between the metallic lithium foil and the carbon material in the presence of the non-aqueous electrolyte, and the metallic lithium foil is electrochemically dissolved to form a local battery. It is considered that the carbon is intercalated in the carbon material and held as lithium that can be discharged in the carbon material, and this is due to discharge during overdischarge.

Therefore, the physical properties and structure of the carbon material do not change, and reversibility in lithium intercalation / deintercalation is not lost. Therefore, after over-discharging, the capacity is quickly restored by recharging, and the capacity deterioration due to the cycle does not change as compared with that before over-discharging. That is, it is considered that good over-discharge resistance characteristics were obtained.

Here, when the sticking capacity of the metallic lithium foil was 4% or more of the saturated reversible capacity of the carbon material, the capacity recovery characteristics were good, but when it was further 40% or more, The capacity recovery characteristic begins to deteriorate. This is because when the sticking capacity of the metallic lithium foil increases, the potential of the positive electrode drops to 1.5 V or less with respect to lithium during over-discharging and becomes equal to the potential of the negative electrode until the battery voltage reaches 0 V. Due to the increased capacity. Therefore, the physical properties and structure of lithium cobalt oxide (LiCoO ₂ ) change, and the intercalation of lithium with the carbon material /
It is considered that the reversibility in deintercalation was lost.

Therefore, when the metal lithium foil is attached to the carbon material to diffuse and hold the dischargeable lithium in the negative electrode, the attachment capacity of the metal lithium foil is the saturated reversible capacity of the carbon material used for the negative electrode material. It is preferably 4 to 40%.

In this embodiment, lithium cobalt oxide (LiCoO ₂ ) was used as the positive electrode active material, but LiMO ₂ or LiM ₂ O ₄ (where M is cobalt, manganese, nickel or iron) was used alone. Alternatively, the same effect was obtained when a lithium-containing composite oxide in which a part of cobalt, manganese, nickel and iron was replaced with another transition metal was used.

In this embodiment, lithium hexafluorophosphate (LiPF ₆ ) is used as the solute of the non-aqueous electrolyte, but other lithium salts such as lithium perchlorate (LiCl ₆ ) are used.
Similar effects were obtained when O ₄ ), lithium hexafluoroarsenate (LiAsF ₆ ), lithium borofluoride (LiBF ₄ ) and the like were used.

Furthermore, in this embodiment, ethylene carbonate (EC) and diethylene carbonate (DEC) were mixed and used in the solvent of the non-aqueous electrolyte, but esters such as propylene carbonate (PC) and butylene carbonate (BC) were used. Similar effects were also obtained when ethers such as etorahydrofuran (THF) were used alone or in combination.

[0046]

As described above, according to the present invention, a lithium-containing oxide of a transition metal is used for the positive electrode, and a carbon material is used for the negative electrode. By sticking a lithium foil and diffusing it into a carbon material due to a potential difference or a concentration difference, and allowing the carbon material of the negative electrode to hold dischargeable lithium, the overdischarge resistance of this battery can be significantly improved.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view showing the structure of a cylindrical lithium secondary battery of the present invention.

FIG. 2 is a view showing the overdischarge behavior of positive and negative electrodes of a conventional battery.

FIG. 3 is a diagram showing the over-discharge resistance characteristics of conventional batteries.

FIG. 4 is a diagram showing overdischarge behavior of positive and negative electrodes of a battery according to the present invention.

FIG. 5 is a diagram showing the relationship between the sticking capacity of metallic lithium and over-discharge resistance of Example 1.

[Explanation of symbols]

 1 Positive Electrode Plate 2 Negative Electrode Plate 3 Separator 4 Positive Electrode Lead Plate 5 Negative Electrode Lead Plate 6 Lower Insulation Ring 7 Battery Case 8 Upper Insulation Plate 9 Assembly Seal Plate

Claims (4)

[Claims] 1. A non-aqueous electrolyte secondary battery in which a lithium-containing composite oxide of a transition metal is used in the positive electrode and a carbon material is used in the negative electrode, and the negative electrode is a metal lithium foil previously attached to the carbon material in potential difference or concentration. A non-aqueous electrolyte secondary battery that is diffused and held in a carbon material due to a difference. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the attachment capacity of the metallic lithium foil is 4 to 40% of the saturation reversible capacity of the negative electrode carbon material. 3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the carbonaceous material of the negative electrode has a lattice spacing (d ₀₀₂ ) determined by a powder X-ray diffraction method of 0.342 nm or less. 4. The lithium-containing composite oxide of the positive electrode is one of the general formulas LiMO ₂ or LiM ₂ O ₄ (where M is cobalt, manganese, nickel or iron) alone, or the above cobalt, manganese, nickel, The non-aqueous electrolyte secondary battery according to claim 1, wherein a part of iron is replaced with another transition metal.