JPH03216960A - Negative electrode for lithium battery - Google Patents

Abstract

PURPOSE:To extract a large current and prevent the deterioration of a negative electrode due to the charge/discharge cycle by providing a composite structure constituted of specific porous carbon and a lithium layer formed on its surface. CONSTITUTION:A composite structure is provided which is constituted of porous carbon with a lithium/carbon layer adsorbing lithium at least on a surface layer section and a lithium layer formed on its surface practically not to block hole sections of the porous carbon. The porous carbon is used for a negative electrode substrate, thus the effective surface area as an electrode is increased, and lithium is added to the carbon of the substrate directly or via aluminum. A large current can be extracted, and the deterioration of the electrode due to the charge/discharge cycle can be prevented.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a negative electrode for lithium batteries, and in particular to a negative electrode that provides a lithium battery that can draw a large current and has high durability against repeated charging and discharging. Regarding.

[Prior art and problems to be solved by the invention] In general, batteries using lithium as a negative electrode active material have advantages such as high energy density (large capacity), being lightweight and compact, and It is widely used as a battery.

In addition, lithium secondary batteries are also being developed, but there are the following problems, and a lithium secondary battery with sufficient performance has not yet been obtained.

First, suitable negative electrode active materials have not been developed.

For example, when a negative electrode active material used in conventional primary batteries is used in a secondary battery, the reproducibility of charge/discharge cycles is poor and the secondary battery does not have sufficient characteristics. this is,
This is because the lithium used as the negative electrode active material tends to generate dendrites due to repeated charging and discharging, and these dendrites tend to react with the electrolyte, so that the negative electrode easily deteriorates.

In addition, although lithium batteries can generally have a high energy density (large capacity), they can only draw a weak current.
Its uses are limited.

Furthermore, if lithium metal were to be used as it is as a negative electrode active material, there would be a problem with safety due to its high reactivity.

Therefore, various attempts have been made to overcome these drawbacks of lithium batteries. For example, there is a method to improve safety and durability of charge/discharge cycles by using a lithium-aluminum alloy instead of lithium as a negative electrode, but even if a negative electrode made of this lithium-aluminum alloy is used, repeated charging and discharging is not possible. If the material is subjected to such damage, it will still deteriorate and will not have a sufficient recycling life. Also,
A lithium secondary battery having a negative electrode in which lithium is occluded in a molded body made of carbon fiber or carbon powder having a specific lattice spacing (Japanese Patent Application Laid-open Nos. 62-268058 and 63)
-102166), but the current that can be extracted from these secondary batteries is not so large and does not meet the purpose of extracting a large current.

Therefore, an object of the present invention is to provide a lithium battery that can draw a large current, does not cause deterioration of the negative electrode even after repeated charging and discharging, has a long recycling life, and has improved safety. The purpose is to provide a negative electrode that can be used.

[Failure to solve the problem]

As a result of intensive research in view of the above objectives, the present inventors have discovered that using porous carbon as a base material, a lithium layer and/or a lithium-containing aluminum layer that does not substantially block the pores of the base material on the surface thereof. By forming a negative electrode with lithium occluded in at least the surface layer of the porous carbon base material, a large current can be extracted and it can have a sufficient lifespan even after repeated charging and discharging.
They also discovered that it is possible to make a lithium battery with improved safety, and completed the present invention.

That is, the first negative electrode for lithium batteries of the present invention includes porous carbon having a lithium/carbon layer in which lithium is occluded at least in the surface layer, and a carbon material that does not substantially block the pores of the porous carbon. and a lithium layer formed on its surface.

Further, the second negative electrode for a lithium battery of the present invention includes porous carbon having a lithium/carbon layer in which lithium is occluded at least in the surface layer, and a porous carbon having a lithium/carbon layer in which lithium is occluded at least in the surface layer, and a porous carbon that does not substantially block the pores of the porous carbon. It is characterized by having a composite structure consisting of an aluminum layer containing lithium formed on its surface.

Furthermore, the third negative electrode for a lithium battery of the present invention includes porous carbon having a lithium/carbon layer in which lithium is occluded at least in the surface layer, and a structure that does not substantially block the pores of the porous carbon. It is characterized by having a composite structure consisting of a lithium layer formed on its surface and an aluminum layer containing lithium.

The present invention will be explained in detail below.

The porous carbon used in the present invention is produced, for example, from carbon fibers, carbon blocks, or artificial graphite, and is not particularly limited to its origin, but is a carbon material that is porous within the range shown below.

That is, the porosity of this porous carbon is 30 to 95%,
Preferably 50-90%, apparent density 0.05-90%
1.2 glcd, preferably 0. 1 ~ 0. 8g
/cd, and the pore diameter is 10JJI to 1, preferably 2
0-300 prisoners.

The first negative electrode for a lithium battery of the present invention uses the above porous carbon as a base material, has a lithium layer on its surface that does not substantially block the pores of the base material, and has at least a surface layer of the base material. is a complex formed by absorbing lithium.

Various methods can be considered for forming a lithium layer on the surface of porous carbon and occluding lithium inside the porous carbon base material. Although not particularly limited, the porous carbon base material and metallic lithium are used as positive and negative electrodes, and lithium Applications include an electrolysis method using an electrolytic solution in which a metal salt is dissolved in an organic solvent, a melting method in which lithium is melted by heating on the substrate, or a dipping method in which the substrate is immersed in molten lithium metal. I can do it.

When forming a lithium layer on the surface of a porous carbon substrate using each of the above methods, lithium spreads to the inside of the substrate in an atomic state, and not only forms a lithium layer on the surface of the substrate.
Lithium is occluded inside the base material. It is most preferable that lithium is occluded throughout the porous carbon base material, but it is sufficient that lithium is occluded to a certain depth from the surface of the base material. Further, the lithium layer formed on the surface of the porous carbon base material may be not only metallic lithium but also an alloy such as lithium-aluminum, and any material containing metallic lithium may be used.

The second negative electrode for a lithium battery of the present invention has an aluminum layer containing lithium on the surface of the above-mentioned porous carbon base material, and lithium is occluded in at least the surface layer of the porous carbon base material.

The aluminum layer containing lithium may be one in which an aluminum layer is formed in advance by a method such as vapor deposition or thermal spraying, and lithium is occluded therein, or it may be a lithium-aluminum alloy.

Methods for occluding lithium in the aluminum layer include, for example, reducing the aluminum layer in a non-aqueous solvent in the presence of lithium perchlorate, and immersing the aluminum layer in molten lithium. Lithium has good diffusivity in aluminum, so by providing an aluminum layer on the surface of the base material and immersing it in molten lithium, lithium can pass through the aluminum layer and reach the interior of the base material. can.

The introduction of aluminum improves the wettability of the base material carbon and lithium, increasing the bonding strength between lithium and the base material, and effectively preventing the formation of lithium dendrites, improving the stability of the electrode. will improve. Furthermore, the presence of aluminum makes it easier to adsorb and occlude lithium into the base material made of carbon.

The third negative electrode for lithium batteries of the present invention has a lithium layer and aluminum containing lithium on the surface of the above-mentioned porous carbon base material, and these two layers are substantially based on the base material. Do not block the holes in the material.

Such a negative electrode can be obtained, for example, by immersing porous carbon whose surface is coated with aluminum in advance in molten lithium. As mentioned above, aluminum easily diffuses lithium, so if porous carbon with an aluminum coating layer is immersed in molten lithium, lithium will easily diffuse into the aluminum layer, and lithium will reach the carbon base material. lithium is absorbed into the base layer (at least in the surface layer of the base material).

By forming a composite of such a lithium layer, an aluminum layer containing lithium, and porous carbon that occludes lithium, the charge/discharge cycle performance can be further improved. In addition, adsorption and storage of lithium can be made easier.

[For production]

The negative electrode for lithium batteries of the present invention uses porous carbon as its base material, which increases the effective surface area of the electrode and enables extraction of a large current.

Also, instead of using lithium itself as a negative electrode,
Porous carbon is used as a base material, and a lithium layer and/or
Alternatively, since an aluminum layer containing lithium is provided and a composite is formed in which lithium is occluded in at least the surface layer of the base material, deterioration of the electrode due to charge/discharge cycles can be prevented.

Furthermore, since lithium will be composited with the aluminum layer or the carbon of the base material, the reaction in the air of the negative electrode,
Or even if a reaction occurs due to contact with an aqueous solution, it will be milder and safety will be improved.

〔Example〕

The present invention will be explained in further detail by the following examples.

Example 1 A porous carbon plate (SG-200, manufactured by Showa Denko ■) with a thickness of 2 mm was cut out from a 40 mm x 40 mm sheet.
It was immersed in molten lithium at 300°C for 30 minutes.

When this was pulled up, excess lithium on the surface was removed to obtain a lithium battery negative electrode having a lithium layer on the porous carbon plate and occluding lithium inside the porous carbon plate.

The above negative electrode and a platinum plate were used as a counter electrode, and a 1:I solution of propylene carbonate/ethylene glycol dimethyl ether in which lithium perchlorate was dissolved at a concentration of 1 mol/1 was used as an electrolyte.

Under the above conditions, the change in current amount (ΔI) with respect to the change in potential (ΔV) was investigated using the potential sweep method for the lithium electrode. Then, the ratio ΔI/ΔV between the two was compared with ΔI/ΔV of a battery of Comparative Example 1 (a battery using lithium metal as the negative electrode and the other things being the same as in Example 1), which will be described later. It is shown in Table 1.

Further, when this battery was subjected to an operation of alternately repeating constant current discharging and charging at 50 m^ 50 times, the reproducibility of the potential was good and the charging/discharging cycle performance was good.

Comparative Example 1 A battery was produced in the same manner as in Example 1 except that metallic lithium was used as the negative electrode.

Regarding the obtained battery, Δ[/△
The value of V was determined. Further, when charging and discharging were repeated by constant current discharge at 50 mA in the same manner as in Example 1, reproducibility was lost after 16 repetitions of charging and discharging.

Example 2 The same porous carbon plate as in Example 1 was prepared using lithium/aluminum (30% by weight aluminum) dissolved at 350°C.
) for 60 minutes to remove excess lithium/aluminum attached to the surface of the porous carbon plate when pulled up, resulting in an aluminum layer containing lithium on the porous carbon plate, which occludes lithium inside. A negative electrode for a lithium battery was obtained.

A battery was produced with the same specifications as Example 1 except for using this negative electrode.

For the obtained battery, ΔI/ΔV was determined in the same manner as in Example 1, and compared with Δr/ΔV of the battery in Comparative Example 1. The results are shown in Table 1.

In addition, charging and discharging were repeated 50 times in the same manner as in Example 1, but
No deterioration in charge/discharge cycle performance was observed.

Example 3 An aluminum layer was formed on the same porous carbon plate as in Example 1 by vapor deposition.

This is reduced with a lithium perchlorate solution (a 1:1 solution of brobylene carbonate/ethylene glycol dimethyl ether is used as a solvent), and an aluminum layer containing lithium is formed on the surface, and lithium is occluded inside the porous carbon plate. A negative electrode for a lithium battery was obtained.

A battery was produced with the same specifications as in Example 1 except that this negative electrode was used.

Regarding the obtained battery, ΔI/ΔV was determined and evaluated in the same manner as in Example 1. The results are shown in Table 1.

Example 4 An aluminum layer was formed by vapor deposition on the surface of the same porous carbon plate as in Example 1.

This is immersed in molten lithium for 30 minutes to remove excess lithium that adhered to the surface when pulled up.The surface has a lithium layer and an aluminum layer containing lithium in order from the top, and lithium is occluded inside. A lithium battery was fabricated using a porous carbon plate.

The obtained battery was evaluated by determining ΔI/ΔV in the same manner as in Example 1. The results are shown in Table 1.

Further, charging and discharging was repeated 50 times in the same manner as in Example 1, but no deterioration in charge/discharge cycle performance was observed.

Comparative Examples 2 and 3 For comparison, a negative electrode using a lithium-aluminum alloy (weight ratio 1:1) (Comparative Example 2), an aluminum plate with lithium added by electrolysis (Comparative Example 3), and others A battery was prepared in the same manner as in Example 1, and the value of Δ■/ΔV was determined in the same manner as in Example 1. This was evaluated by comparing it with that of Comparative Example 1. The results are shown in Table 1.

Table 1 Note 1》Relative value when △I/△V in Comparative Example 1 is set to 1.

2) Number of repetitions of 50 mA constant current discharge and charge.

3) Not measured.

〔Effect of the invention〕

According to the present invention, since porous carbon is used as the negative electrode base material, the effective surface area of the electrode increases, thereby making it possible to extract a large current.

Furthermore, since the base material is carbon and lithium is added to it either directly or via aluminum, deterioration of the negative electrode can be prevented even when charging and discharging are repeated, and the charging and discharging cycle performance is improved. Also, since it has the above structure,
Even when the electrode comes into contact with air or water, the reactivity of lithium is reduced, improving safety.

Therefore, the negative electrode of the present invention can extract a large current,
A lithium secondary battery with improved charge/discharge cycle performance and safety can be provided.

Claims (3)

[Claims] (1) Porous carbon having a lithium/carbon layer in which lithium is occluded at least in the surface layer, and a lithium layer formed on the surface so as not to substantially block the pores of the porous carbon. A negative electrode for a lithium battery, characterized by having a composite structure. (2) Contains porous carbon having a lithium/carbon layer in which lithium is occluded at least in the surface layer, and lithium formed on the surface so as not to substantially block the pores of the porous carbon. A negative electrode for a lithium battery characterized by having a composite structure consisting of an aluminum layer. (3) Porous carbon having a lithium/carbon layer in which lithium is occluded at least in the surface layer, and a lithium layer and lithium formed on the surface so as not to substantially block the pores of the porous carbon. A negative electrode for a lithium battery, characterized by having a composite structure consisting of an aluminum layer containing.