JP3063320B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

In recent years, with remarkable portability and intelligence of cordless information and communication devices such as mobile phones and camcorders, small and lightweight secondary batteries with high energy density have been demanded as power supply batteries for driving the devices. 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 very large.

[0003]

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

[0004] 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 does not precipitate on the electrode during charging, which is expected to improve safety and at the same time, is considered to be excellent in quick charging characteristics, and research and development are being actively conducted at present.

In this battery, the active material of the positive electrode is 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, Manganese, nickel iron, etc.) have attracted 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 considered to be promising for deintercalation, and the reversibility of the intercalation / deintercalation and the relationship between the physical properties and structure of the carbon material are being actively studied.

[0007]

As described above, by using a lithium-containing composite oxide of a transition metal as a positive electrode active material and a carbon material as a negative electrode material, a small and light-weight, high-safety material having excellent safety can be obtained. It is considered that a non-aqueous electrolyte secondary battery having an energy density can be provided.

However, this battery still has some problems. One of them is an improvement in overdischarge resistance.

Recent cordless information / communication devices are often equipped with a so-called automatic power-off function in order to save power batteries. This function is performed when the power is on, (1) the device is not driven, that is, when a certain period of time has elapsed in a so-called pause state, and (2) when the device is driven and the battery voltage reaches the set lower limit voltage. The power is automatically turned off.

[0010] If the auto power-off function is operated and the battery is further left, the battery continues to be discharged by the circuit load, and the battery voltage eventually reaches 0V. Therefore, even if the battery is recharged even after such overdischarge, the capacity is restored. If the battery is not excellent in so-called overdischarge resistance, the practicality of the battery is extremely low.

However, in the case of a non-aqueous electrolyte secondary battery using a lithium-containing composite oxide of a transition metal as the active material of the positive electrode and a carbon material as the negative electrode material, recharging is performed after such overdischarge. It was also found that the capacity hardly recovered, and that the capacity deterioration due to the cycle was much greater than 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, depending on the overlap of the layers in the c-axis direction and the spread of the layers in the a-axis direction.

However, when this battery is over-discharged, the potential of the negative electrode rises to about 3.2 V or more with respect to lithium, becomes equal to the potential of the positive electrode, and the battery voltage reaches 0 V. Was.

For this reason, the physical properties and structure of the carbon material change, and the reversibility of lithium intercalation / deintercalation is lost. After overdischarge, the capacity is almost recovered even if recharged. It is considered that the capacity deterioration due to the cycle is extremely large as compared with that before the overdischarge.

The present invention has been made to solve this problem, and has as its object to improve the overdischarge resistance of a lithium secondary battery.

[0016]

SUMMARY OF THE INVENTION The present invention is a non-aqueous electrolyte secondary battery in which a lithium-containing composite oxide of a transition metal is used for a positive electrode and a carbon material is used for a negative electrode. The dischargeable lithium is held in the carbon material of the negative electrode by diffusing the metallic lithium foil into the carbon material by a potential difference or a concentration difference.

Further, the adhering capacity of the metallic lithium foil is 4 times the saturated reversible capacity of the carbon material used for the negative electrode material.
４０40%.

Here, the saturation reversible capacity of the carbon material used for the negative electrode material was calculated by the following method. Using carbon material for the cathode material and metallic lithium for the anode material, respectively,
Five cycles of constant current charging / discharging at a current density of 0.5 mA / cm ² at ℃ were repeated. The capacity at this time was defined as a saturated reversible capacity. The upper limit voltage during charging was 1.0 V, and the lower limit voltage during discharging was 0 V.

In addition, the active material of the positive electrode has the general formula LiMO
₂ or LiM ₂ O ₄ (where M is cobalt, manganese,
Nickel or iron) 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 powder X-ray diffraction A carbon material having a lattice spacing (d ₀₀₂ ) of 0.342 nm or less is preferred.

[0020]

According to the present invention, the metal lithium foil adhered to the carbon material of the negative electrode forms a local battery with the carbon material in the presence of the non-aqueous electrolyte, and the metal lithium is dissolved electrochemically. It is intercalated into a nearby carbon material and is held in the carbon material as dischargeable lithium.

The potential of the negative electrode does not increase due to the discharge of lithium retained in the carbon material during overdischarge, and therefore the physical properties and structure of the carbon material do not change.
Reversibility in intercalation / deintercalation of lithium to carbon material is not lost. Therefore, even in a battery after overdischarge, the capacity is quickly recovered by recharging, and the capacity deterioration due to the cycle does not change compared to that before the overdischarge. That is, the overdischarge resistance can be improved.

[0022]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a longitudinal 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 cobalt trioxide (Co ₃ O ₄ ), and calcined in air at 900 ° C.
O ₂ ) as an active material, 3% by weight of acetylene black as a conductive agent was mixed with the active material, and then polytetrafluoride was used as a binder with an aqueous dispersion of polytetrafluoroethylene resin .
A mixture prepared by kneading 7% by weight of ethylene resin into a paste is applied to both sides of a core material made of aluminum foil, dried and rolled. The positive electrode lead plate 4 is spot-welded to the end. The dimensions of this positive electrode plate are 40m in width.
m, length 250 mm, thickness 0.170 mm.

Further negative electrode plate 2, a mesophase pitch spheroidal graphite heat treated at 2800 ° C. under an argon atmosphere, an aqueous dispersion with polytetrafluoroethylene resin polytetrafluoroethylene resin as a binder 5 wt%
The mixture obtained by kneading into a paste is applied to both sides of a core material made of copper foil, dried and rolled. A negative electrode lead plate 5 is spot-welded to the end. The dimensions of the negative electrode plate are 42 mm in width and 270 mm in length, and the thickness is 0.20 mm.
5 mm.

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

As the separator 3, a porous film made of polypropylene, which was cut more widely than the positive electrode plate and the negative electrode plate, was used.

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

Next, the upper and lower portions of the electrode plate group are heated with warm air to thermally shrink the separator 3. The lower insulating ring 6 is attached to the lower side of the electrode plate group, accommodated in the battery case 7, and spot-welded the negative electrode lead plate 5 to the battery case 7. An upper insulating ring 8 is mounted on the upper side of the electrode plate group, and a battery case 7 is provided.
Then, a non-aqueous electrolyte is injected. The non-aqueous electrolyte is prepared by mixing 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 spot welding the assembled sealing plate 9 in which the gasket was previously incorporated and the positive electrode lead plate 4, the assembled sealing plate 9 was mounted on the battery case 7 and swaged to form a battery. The dimensions of this battery are outer diameter 14mm, total height 50mm
(AAA).

Test Evaluation The overdischarge resistance of the battery constructed 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 4.1 V, and the lower limit voltage during discharging was 3.0 V. Thereafter, the battery was changed from the discharged state to the over-discharged state, and the constant-resistance discharge of 1 kΩ was further continued for 2 weeks. At this time, the results of observing the overdischarge behavior of the positive and negative electrodes using metallic lithium as the reference electrode are shown in FIG. Then, the constant current discharge at 100 mA was repeated 50 cycles. FIG. 3 shows the results of comparing the capacity recovery characteristics and the cycle characteristics before and after overdischarge.

As apparent from FIG. 3, it was found that the capacity recovered only about 55% after overdischarge, and that the capacity deterioration accompanying the cycle was significantly larger than before the overdischarge.

In normal charging and discharging, the potential of the positive electrode is
It is considered that there is no problem because it is near, but the potential of the negative electrode is about
It is from 0.1 V (during charging) to about 0.5 V (during discharging).

As shown in FIG. 2, the potential of the negative electrode
It rises to 3.2V or more with respect to lithium when
It is equal to the potential of the pole, and the battery voltage has reached 0V
I understand.

[0033]

As a result, the physical properties and structure of the carbon material change, and the reversibility of lithium intercalation / deintercalation is lost. As a result, the capacity is almost recovered even after overdischarge and recharging. It is considered that the capacity deterioration due to the cycle becomes extremely large as compared with that before the overdischarge.

Example 1 A battery was constructed in the same manner as described above using a negative electrode plate in which a metallic lithium foil was previously adhered to a carbon material, and the overdischarge resistance was evaluated. As an example, FIG. 4 shows the result of observing the overdischarge behavior of the positive and negative electrodes when the attached capacity of the metal lithium foil was set to 20% of the saturated reversible capacity of the carbon material. At this time, the dimensions of the metallic lithium foil were 40 mm wide, 40 mm long, and 0.0 mm thick.
It was 30 mm.

FIG. 5 shows the relationship between the attached capacity of the metal lithium foil and the capacity recovery property as the overdischarge resistance property. At this time, the dimensions of the metallic lithium foil were 40 mm wide and 40 mm long.
And the capacity was adjusted by the thickness.

As is apparent from FIG. 5, when the adhering capacity of the metal lithium foil is set to 4% or more of the saturated reversible capacity of the carbon material, a better overdischarge 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 only increased to about 1.5 V with respect to lithium at the time of overdischarge. Furthermore, it was confirmed that the same overdischarge behavior was obtained when the attached capacity of the metal lithium foil was 4% or more of the saturated reversible capacity of the carbon material.

This is because the lithium metal foil adhered to the negative electrode carbon material forms a local battery with the carbon material in the presence of the non-aqueous electrolyte, and the lithium metal foil is dissolved electrochemically in the vicinity thereof. It is considered that lithium was intercalated in the carbon material and was held as dischargeable lithium by the carbon material, and this was discharged at the time of overdischarge.

Therefore, the physical properties and structure of the carbon material do not change, and reversibility in lithium intercalation / deintercalation is not lost. Therefore, the capacity is quickly recovered by recharging after the overdischarge, and the capacity deterioration due to the cycle does not change compared to that before the overdischarge. That is, it is considered that good overdischarge resistance was obtained.

Here, when the adhering capacity of the metal lithium foil was 4% or more of the saturated reversible capacity of the carbon material, the capacity recovery characteristics were good. The capacity recovery characteristics begin to deteriorate. This is because when the attached capacity of the metal lithium foil increases, the potential of the positive electrode falls to 1.5 V or less with respect to lithium at the time of overdischarge and becomes equal to the potential of the negative electrode until the battery voltage reaches 0 V. Due to the increase in capacity. For this reason, the physical properties and structure of lithium cobaltate (LiCoO ₂ ) change, and the intercalation /
It is considered that reversibility in deintercalation was lost.

Therefore, when the dischargeable lithium is diffused and held in the negative electrode by attaching the metallic lithium foil to the carbon material, the attached capacity of the metallic lithium foil is larger than the saturated reversible capacity of the carbon material used for the anode material. It is preferably 4 to 40%.

[0043] Although using the lithium cobaltate (LiCoO ₂₎ as a positive electrode active material in the present embodiment, LiMO ₂ or LiM ₂ O ₄ (where M is cobalt, manganese, nickel, or iron), and alone Alternatively, a similar effect was obtained when a lithium-containing composite oxide in which 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 a solute of the nonaqueous electrolyte. However, other lithium salts, for example, lithium perchlorate (LiCl ₆ ) are used.
O ₄ ), lithium hexafluoroarsenate (LiAsF ₆ ), lithium borofluoride (LiBF ₄ ), and the like provided almost the same effect.

Further, in this embodiment, ethylene carbonate (EC) and diethylene carbonate (DEC) are used as a mixture in the solvent of the non-aqueous electrolyte, but esters such as propylene carbonate (PC) and butylene carbonate (BC) are used. The same effect was obtained when ethers such as triethylhydrofuran (THF) and the like were used alone or in combination.

[0046]

As described above, according to the present invention, in a non-aqueous electrolyte secondary battery using a lithium-containing oxide of a transition metal for the positive electrode and a carbon material for the negative electrode, By sticking a lithium foil and diffusing the lithium foil into a carbon material by a potential difference or a concentration difference and holding the dischargeable lithium in the carbon material of the negative electrode, the overdischarge resistance of the battery can be remarkably improved.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing a configuration of a cylindrical lithium secondary battery of the present invention.

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

FIG. 3 is a diagram showing overdischarge resistance characteristics of a conventional battery;

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

FIG. 5 is a diagram showing the relationship between the adhered capacity of metallic lithium and overdischarge resistance of Example 1.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Positive electrode plate 2 Negative electrode plate 3 Separator 4 Positive electrode lead plate 5 Negative electrode lead plate 6 Lower insulating ring 7 Battery case 8 Upper insulating plate 9 Assembly sealing plate

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-4-188559 (JP, A) JP-A-64-14881 (JP, A) JP-A-4-229561 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) H01M 10/40 H01M 4/02-4/04 H01M 4/58

Claims (3)

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