JPH10144295A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery which efficiently stores lithium ions in negative electrode active material, and can thereby surely supplement loss in capacity in the negative electrode. SOLUTION: This secondary battery is equipped with a positive electrode 1 containing lithium ions, a negative electrode 2 capable of storing and emitting lithium ions, and with non-aqueous electrolyte. In this case, carbon material 18 used as active material for the negative electrode 2, a conductive metal 15 capable of forming no alloy with lithium, is deposited on the surface of the carbon material to the thickness less than 1μm, and furthermore, metal lithium 16 is deposited on the surface of the conductive metal 15.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium ion secondary battery comprising a positive electrode containing lithium ions, a negative electrode made of a carbon material capable of inserting and extracting lithium ions, and a non-aqueous electrolyte.

[0002]

2. Description of the Related Art Conventionally, lithium ion secondary batteries have been expected to be greatly developed because of their large discharge capacity, high voltage and high energy density.

In this lithium ion secondary battery, during charging, lithium ions are released from the positive electrode, and the lithium ions are occluded in the negative electrode. When discharging, charging and discharging are performed by the reverse operation. Lithium ions to be occluded, or lithium ions occluded in the negative electrode, are consumed in the decomposition reaction of the electrolytic solution,
Alternatively, there is a phenomenon in which the amount of lithium ions released from the negative electrode at the time of discharge becomes smaller than the amount of occluded lithium ions by producing a compound with a carbon material (this is referred to as negative electrode loss capacity). This negative electrode loss capacity is one of the factors that hinder the increase in capacity of the secondary battery.

Therefore, conventionally, in order to compensate for the above-mentioned negative electrode loss capacity, metallic lithium is directly adhered to the negative electrode active material, and this negative electrode active material is put into an electrolytic solution, and a potential difference and a concentration difference generated at that time are caused. Thus, a method of inserting lithium ions into the negative electrode active material has been attempted.

[0005]

However, according to such a method, it takes not only long time to occlude lithium ions in the negative electrode active material, but also the density of metallic lithium adhering to the surface of the negative electrode active material. Unevenness occurs,
Metallic lithium is left undissolved, and as a result,
There was a problem that the charge / discharge cycle characteristics were affected.

SUMMARY OF THE INVENTION The present invention has been made in view of such a conventional problem. A lithium ion secondary battery capable of efficiently absorbing lithium ions into a negative electrode active material and reliably compensating for the loss capacity of the negative electrode. Providing a battery is an issue to be solved.

[0007]

According to a first aspect of the present invention, there is provided a lithium ion secondary battery comprising: a positive electrode containing lithium ions; a negative electrode capable of inserting and extracting lithium ions; In a lithium ion secondary battery provided with a non-aqueous electrolyte, a carbon material is used as an active material of the negative electrode, and a conductive metal that does not form an alloy with lithium has a thickness of 1 μm or less on the surface of the carbon material. It is characterized in that it is deposited, and further, metal lithium is deposited on the surface of the conductive metal.

A lithium ion secondary battery according to a second aspect of the present invention is characterized in that, in the first aspect, the conductive metal that does not form an alloy with lithium is copper.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a longitudinal sectional view of a lithium ion secondary battery according to the present embodiment. In the lithium ion secondary battery, a sheet-like positive electrode plate 1 and a negative electrode plate 2 are overlapped with a film-like separator 3 interposed therebetween. A spiral electrode 4 formed by spirally winding them, a bottomed cylindrical outer can 5 also serving as a negative electrode terminal in which the spiral electrode is housed, and an opening in the outer can 5 insulated. A sealing plate 7 attached via a gasket 6 to seal the opening;
A positive electrode terminal 8 integrated by spot welding or the like;
A sheet-shaped safety valve 9 that is interposed between the positive electrode terminal 8 and the sealing plate 7 and that discharges internal gas to the outside when the internal pressure of the battery increases, and the positive electrode plate 1 and the sealing plate 7 It is constituted by a positive electrode lead terminal 10 that conducts electricity and a negative electrode lead terminal 11 that conducts electricity between the negative electrode plate 2 and the outer can 5.

More specifically, the positive electrode plate 1 uses LiMn 2 O 4 as a positive electrode active material, carbon powder as a conductive material, and an aqueous dispersion of polytetrafluoroethylene (hereinafter referred to as PTFE) as a binder. These were mixed at a weight ratio of 100: 10: 10, kneaded in a paste with water, applied to both sides of a 30-μm-thick aluminum foil as a current collector, dried and rolled, The mixture was integrally formed on both surfaces of the current collector, and then cut into a predetermined size to form a sheet.

Here, the aqueous dispersion of PTFE was a solid content ratio, and the weight of the positive electrode active material was 4.7 g.

In the positive electrode plate 1, the current collector is exposed by scraping a part of the mixture so as to be orthogonal to the length direction, and the exposed portion of the positive electrode lead terminal 10 are attached by spot welding.

The negative electrode plate 2 uses artificial graphite as a carbon material as a negative electrode active material, and uses polyvinylidene fluoride (hereinafter, referred to as PVDF) as a binder to bind to these negative electrode active materials. And a mixing agent at a weight ratio of 100: 5 (here, the weight of the artificial graphite as the negative electrode active material was 2.1 g), and an NMP solvent was added and kneaded to form a slurry. Thickness of slurry as current collector 1
After coating and drying on one side of a copper foil 13 of 0 μm to form a mixture 14 integrally, the mixture 14 is placed in a vapor deposition apparatus and the mixture 14 is placed.
Surface (that is, the surface of artificial graphite that is the negative electrode active material)
In addition, copper element 15 as a metal that does not form an alloy with lithium
Is deposited to a thickness of 1 μm or less, and further, metal lithium 16 is deposited on the surface of the deposited copper in an amount corresponding to an assumed negative electrode loss capacity, and cut into a predetermined size to form a sheet. Yes (see FIG. 3), a negative electrode is formed by stacking two sheets.

The copper 15 and the metallic lithium 16
Was deposited using resistance heating to heat the copper 15 and the metal lithium 16 under the conditions of a degree of vacuum of 10 ⁻⁶ Torr or less, and the film thickness was measured with a microinterferometer.

In the negative electrode plate 2, the current collector is exposed by scraping a part of the mixture so as to be orthogonal to the length direction, and the exposed portion of the negative electrode lead terminal 11 is attached by spot welding.

On the other hand, each of the positive electrode plate 1 and the negative electrode plate 2 configured as described above is spirally wound with the separator 3 interposed therebetween as shown in FIG. In addition to being inserted into the outer can 5, an insulating bottom plate 12 is provided at the bottom thereof, and the positive lead terminal 10 is connected to the sealing plate 7 and the negative lead terminal 11 is connected to the inner bottom surface of the outer can 5. Each is connected by spot welding.

In the outer can 5 into which the spiral electrode 4 is inserted as described above, 1 mol / l of LiPF6 is added as an electrolyte to an organic solvent in which ethylene carbonate and diethyl carbonate are mixed at a ratio of 1: 1. 2.3 ml of the injected electrolyte solution is injected, the insulating gasket 6 is attached to the outer periphery, and the sealing plate 7 to which the positive electrode terminal 8 is attached is installed inside the outer can 5 so as to cover the opening. After that, the opening edge portion of the outer can 5 is swaged inward over the entire circumference to fix the sealing plate 7 to the outer can 5 via the insulating gasket 6 and the insulating gasket. 6 is compressed, the opening of the outer can 5 is hermetically sealed, and the lithium ion secondary battery according to the present embodiment is assembled.

With respect to the thus assembled lithium ion secondary battery according to the present embodiment, after assembling, storing it at a temperature of 45 ° C. for 5 days, charging at a constant current and a constant voltage (a charging current of 0.5 C and a charging voltage of 4 C) .2 V) and constant current discharge (discharge current 0.5 C, discharge voltage 3.0 V) for 100 cycles, and the initial charge capacity and initial discharge capacity were measured. The results shown in Table 1 were obtained. When the cycle characteristics were measured, the results shown in FIG. 2 were obtained.

[Table 1] Further, for comparison, lithium ion secondary batteries were prepared under the conditions shown in Comparative Examples 1 to 3 below, and the initial charge capacity and the initial discharge capacity were measured under the same conditions as in the present embodiment, and the cycle characteristics were measured. The results are shown in Table 1 and FIG.
In addition, each comparative example and this embodiment differ only in the structure of the negative electrode, and the other parts are exactly the same as the configuration shown in the present embodiment.

Comparative Example 1 Artificial graphite was used as an active material of a negative electrode, and P was used as a binder.
Using a VDF, an NMP solvent is added thereto and kneaded to form a slurry. The slurry is applied to both sides of a copper foil having a thickness of 20 μm and dried to form a negative electrode. A secondary battery is formed using the negative electrode. It was created.

Comparative Example 2 Artificial graphite was used as the active material of the negative electrode, and P was used as the binder.
Using a VDF, an NMP solvent was added to these and kneaded to prepare a slurry. The slurry was applied to one surface of a copper foil having a thickness of 10 μm, and dried. After evaporation to the thickness, on the surface,
Metallic lithium is deposited by the amount corresponding to the negative electrode loss capacity.
A secondary battery was formed as a negative electrode by stacking the sheets.

Comparative Example 3 Artificial graphite was used as the active material of the negative electrode, and P was used as the binder.
Using a VDF, an NMP solvent was added thereto and kneaded to prepare a slurry. The slurry was applied to one surface of a copper foil having a thickness of 10 μm and dried. A secondary battery was formed as a negative electrode by depositing two electrodes directly on the surface of the active material.

As is clear from the results shown in Table 1 above,
The lithium ion secondary battery according to the present embodiment has an initial discharge capacity / initial charge capacity of 2.7% or more as compared with each comparative example.
An improvement of 6.4% is seen, and the charge / discharge efficiency is greatly improved.

As is apparent from the results shown in FIG. 2, the lithium-ion secondary battery according to the present embodiment has almost stable discharge characteristics despite the increase in the number of times of charging and discharging. Capacity is secured.

As described above, the high charge-discharge efficiency is obtained in the present embodiment, the battery capacity is large, and the charge-discharge cycle characteristics are greatly improved. By vapor deposition below, by depositing metal lithium 16 on the surface of the copper element 15, the metal lithium 16 is uniformly dispersed by the copper element 15 and deposited on the surface of the negative electrode active material. When the thickness of the deposited film of the copper element 15 is 1 μm or less, conduction between the negative electrode active material and metallic lithium is performed well, and lithium ions are smoothly dissolved into the negative electrode active material. And so on.

That is, in comparison between Comparative Example 1 in which lithium ion was not compensated for the negative electrode loss capacity, Comparative Example 3 in which only metal lithium was deposited, and Example, Comparative Example 3 was different from Comparative Example 1 in the number of charge / discharge cycles. Since the discharge capacity is large in the region where the amount is small, and the discharge capacity is large irrespective of the number of times of charging and discharging in the embodiment, it is assumed that the supplementation of lithium ions from the metal lithium 16 contributes to the increase in the battery capacity. Is done.

In Comparative Example 3, the discharge capacity decreased as the number of times of charging and discharging increased, and the discharge capacity became smaller than that in Comparative Example 1. When directly vapor-deposited on a portion of the deposited metal lithium film, a portion where conduction between metal lithium and carbon is poor occurs, and the metal lithium at the portion where the conduction is poor does not ionize, and the lithium metal is not ionized in the negative electrode active material. It is supposed that the metal lithium remains without being dissolved and the remaining metal lithium lowers the discharge capacity.

This means that, in the embodiment, the metallic lithium and the negative electrode active material are replaced by a copper element 15 which does not form an alloy with lithium.
, The discharge capacity is generally increased irrespective of the charge / discharge cycle, and the function of the copper element 15 is to improve the conduction state between the metal lithium 16 and carbon.

Further, as in Comparative Example 2, the thickness of the copper
When the thickness exceeds μm, the discharge amount is reduced, and it is considered that the inflow and out of lithium ions are suppressed because the carbon surface is covered with the copper element.

The above-described embodiment is merely an example, and various modifications can be made based on design requirements and the like. For example, in the above-described embodiment, the cylindrical secondary battery is shown. However, other shapes such as a button shape may be used, and copper 15 may be used as a metal that does not form an alloy with lithium.
However, this is adopted by paying attention to the fact that the copper 15 is particularly excellent in conductivity, and may be changed to another metal depending on the type of the battery or the like.

[0030]

As described above, according to the first aspect of the present invention,
According to the lithium ion secondary battery according to the above, a conductive metal that does not form an alloy with lithium is 1 μm
By depositing the following thickness and depositing metallic lithium on the surface of the conductive metal, the conduction state between metallic lithium and the negative electrode active material is improved, and lithium ions corresponding to the loss capacity of the negative electrode are used. Into the negative electrode active material smoothly and reliably, thereby increasing the initial charge and discharge efficiency and increasing the battery capacity,
Furthermore, charge / discharge cycle characteristics can be improved.

According to the lithium ion secondary battery of claim 2 of the present invention, by using copper as the conductive metal that does not form an alloy with lithium in claim 1, the metallic lithium and the negative electrode active material And the operation and effect obtained in claim 1 can be further ensured.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a lithium ion secondary battery according to an embodiment of the present invention.

FIG. 2 is a charge / discharge cycle characteristic diagram of a lithium ion secondary battery according to one embodiment of the present invention and a conventional lithium ion secondary battery.

FIG. 3 is an enlarged vertical sectional view of a negative electrode plate.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Positive electrode plate 2 Negative electrode plate 15 Copper (conductive metal) 16 Metal lithium

Claims (2)

[Claims] 1. A lithium ion secondary battery comprising a positive electrode (1) containing lithium ions, a negative electrode (2) capable of inserting and extracting lithium ions, and a non-aqueous electrolyte, the active material of the negative electrode (2) A conductive metal (1) that does not form an alloy with lithium is used on the surface of the carbon material.
5) A lithium ion secondary battery characterized in that (5) is deposited to a thickness of 1 μm or less, and further, metal lithium (16) is deposited on the surface of the conductive metal (15). 2. The lithium ion secondary battery according to claim 1, wherein the conductive metal that does not form an alloy with lithium is copper.