JP3728831B2 - Lithium secondary battery - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery that suppresses the deposition of lithium metal and lithium compound dendrites on the negative electrode surface.
[0002]
[Prior art]
There is a need for technology that makes more effective use of electric power against the background of energy problems and environmental problems. For this purpose, an excellent electricity storage means is necessary, and it is optimal to use a high-performance secondary battery having a large discharge capacity and capable of repeated charge and discharge.
[0003]
Under these circumstances, occlusion of lithium, a lithium secondary battery using a positive electrode and the negative electrode is capable release is used as a battery such as a small portable electronic device because it has a very large discharge capacity, further Expectations for its use as a battery for vehicles such as electric vehicles are also increasing.
However, in a conventional lithium secondary battery, by repeating charging and discharging, lithium metal and a lithium compound are deposited on the surface of the negative electrode, and this precipitate grows greatly with the passage of time, and dendrites are generated. This dendrite deteriorates the cycle characteristics by changing the electrical properties of the negative electrode surface and reducing the discharge capacity of the lithium secondary battery.
[0004]
Further, when the dendrite grows greatly, it finally penetrates the separator, and this dendrite causes a short circuit between the positive electrode and the negative electrode. Due to this short circuit, the battery may become inoperable or may operate abnormally. This suddenly stops the device in use, or causes malfunction or destruction of the device.
[0005]
Furthermore, when using a non-aqueous electrolyte, the lithium metal of the negative electrode reacts with the non-aqueous electrolyte and the discharge capacity of the lithium secondary battery is reduced, thereby deteriorating the cycle characteristics. Further, gas is generated along with this reaction. Due to the generated gas, the internal pressure in the battery is increased, the battery container is ruptured, and there is a risk of liquid leakage. Therefore, the generation of such gas becomes a problem in terms of safety as well as the generation of dendrites.
[0006]
[Problems to be solved by the invention]
The present invention has been made in view of the above circumstances, and suppresses the generation of dendrites of lithium metal and lithium compounds on the negative electrode surface. In a lithium secondary battery having a non-aqueous electrolyte, the lithium metal of the negative electrode An object of the present invention is to provide a lithium secondary battery having excellent cycle characteristics and high safety by preventing reaction with a water electrolyte.
[0007]
[Means for Solving the Problems]
The present inventor has found that, in a conventional lithium secondary battery, by covering the surface of the negative electrode with a lithium oxide film, generation of dendrites of lithium metal and lithium compound on the negative electrode surface can be suppressed, leading to the present invention. It is a thing.
[0008]
[Means for Solving the Problems]
That is, the lithium secondary battery of the present invention is a secondary battery having a non-aqueous electrolyte and having a negative electrode made of lithium metal or a lithium alloy, and the non-aqueous electrolyte contains hydrogen peroxide capable of oxidizing lithium. It is characterized by containing 10 ⁻⁴ mol / l or more and 10 ⁻¹ mol / l or less.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
In the lithium secondary battery of the present invention, the negative electrode made of lithium metal or lithium alloy is oxidized with hydrogen peroxide capable of oxidizing lithium. By this oxidation treatment, a lithium oxide film is formed on the entire surface of the negative electrode.
[0010]
In this lithium secondary battery, the oxidizing agent capable of oxidizing lithium is hydrogen peroxide. Thereby, the chemical reaction shown by the following chemical reaction formula 1 occurs on the negative electrode surface, and an oxide film is formed on the negative electrode surface.
[0011]
[Chemical 1]
H ₂ O ₂ → H ₂ O + 1 / 2O ₂
2Li + 1 / 2O ₂ → Li ₂ O
This lithium oxide film suppresses the formation of dendrites of lithium metal and lithium compounds on the negative electrode surface during charging. Moreover, when using a non-aqueous electrolyte, it is made for the electrolyte solution intervening between a positive electrode and a negative electrode not to contact the surface of a negative electrode directly.
[0012]
The film thickness of this lithium oxide film is not particularly limited. Further, the uniformity of the film thickness is not particularly limited, but it is desirable that the film thickness is uniform from the viewpoint of the efficiency of the electrode reaction.
In this lithium secondary battery, either an electrolytic solution or a solid electrolyte may be used as the electrolyte interposed between the positive electrode and the negative electrode. In this case, when using an electrolytic solution, a nonaqueous electrolytic solution in which a supporting salt containing lithium is dissolved in an organic solvent having a high dielectric constant and a low viscosity can be used. Examples of the organic solvent include propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), dimethoxyethane (DME), tetrahydrofuran (THF), 2-methyltetrahydrofuran (2MeTHF) dioxolane and the like. At least one of the above can be used. As the supporting salt, LiPF ₆ , LiClO ₄ , LiBF ₄ , Li (CF ₃ SO ₂ ) N ₂ , Li (CF ₃ SO ₂ ) ₃ C, or the like can be used. On the other hand, when a solid electrolyte is used, polyethylene oxide, polyacrylonitrile, acrylic, fluoropolymer, etc., and those in which the supporting salt is included in these dielectrics can be used.
[0013]
Further, the shape of the lithium secondary battery is not particularly limited, and can be a coin shape, a button shape, a cylindrical shape, a square shape, or the like depending on the application. Also, the size is not particularly limited and can be selected according to the application. Moreover, the lithium secondary battery of this invention has a non-aqueous electrolyte containing the oxidizing agent which can oxidize lithium. In this lithium secondary battery, an oxide film is formed on the surface of the negative electrode by the oxidizing agent in the non-aqueous electrolyte.
[0014]
Even in this lithium secondary battery, the oxidizing agent capable of oxidizing lithium is hydrogen peroxide. Thereby, the chemical reaction shown by Chemical Reaction Formula 1 occurs on the negative electrode surface as described above, and an oxide film is formed on the negative electrode surface. This lithium oxide film suppresses the formation of dendrites of lithium metal and lithium compounds on the negative electrode surface during charging, as in the above-described invention. Moreover, when using a non-aqueous electrolyte, it is made for the electrolyte solution intervening between a positive electrode and a negative electrode not to contact the surface of a negative electrode directly.
[0015]
In this lithium secondary battery, the content of hydrogen peroxide in the non-aqueous electrolyte is 10 ⁻⁴ mol / l or more and 10 ⁻¹ mol / l or less . With this content, an oxide film can be formed with a uniform film thickness, generation of dendrites can be sufficiently suppressed, and high charge / discharge efficiency can be maintained. If the content is less than 10 ⁻⁴ mol / l, an oxide film with a uniform film thickness cannot be formed, and the formation of dendrites may not be sufficiently suppressed. On the other hand, when it exceeds 10 ⁻¹ mol / l, the charge / discharge efficiency is lowered and the cycle characteristics may be shortened.
[0016]
[Action]
In the lithium secondary battery of the present invention, since it is possible to suppress the formation of dendrites on the negative electrode surface during charging, even if charging and discharging are repeated many times, the electrical properties of the negative electrode surface do not change, and the discharge capacity, etc. Will not drop. In addition, there is no short circuit between the positive electrode and the negative electrode. For this reason, the lithium secondary battery of the present invention has excellent cycle characteristics and high safety.
[0017]
Even when a non-aqueous electrolyte is used as the electrolyte, the reaction between the lithium metal contained in the negative electrode and the non-aqueous electrolyte is prevented because the non-aqueous electrolyte is not in direct contact with the surface of the negative electrode. The non-aqueous electrolyte does not decompose and no gas is generated.
In particular, in another lithium secondary battery of the present invention, a lithium oxide film is formed with a uniform film thickness on the negative electrode surface not only during battery formation but also during battery use. For example, during use of this lithium secondary battery, if the film is partially lost due to some reason such as peeling of the lithium oxide film from the negative electrode surface, a lithium oxide film is formed on the part where the film is partially lost. It is formed. For this reason, it is possible to suppress the formation of dendrite on the negative electrode surface during charging even in a part where the film is partially lost, and to prevent the non-aqueous electrolyte from directly contacting the negative electrode surface. it can.
[0018]
【Example】
Hereinafter, the present invention will be described specifically by way of examples.
(Example 1)
As shown in FIG. 1, the lithium secondary battery of this example is assembled so as to have a working electrode 1 as a negative electrode, a counter electrode 2 as a positive electrode, and a non-aqueous electrolyte 4 as an electrolyte. is there.
[0019]
In this lithium secondary battery 10, the working electrode 1 and the counter electrode 2 are made of lithium metal. The non-aqueous electrolyte 4 is prepared by dissolving LiCF ₃ SO ₃ at a concentration of 1 mol / l in propylene carbonate (PC) and further containing hydrogen peroxide at a content of 1.0 × 10 ⁻² mol / l. It was prepared by adding to The working electrode 1 and the counter electrode 2 are each formed integrally with a current collector 5 made of stainless steel, and are fixedly held by an insulator 6. The working electrode 1 and the counter electrode 2 are attached with an electrode performance evaluation charging / discharging device 7 for charging / discharging at a constant current density. The electrode performance evaluation charging / discharging device 7 is provided with a reference electrode 3 capable of measuring a comparison potential of the working electrode 1 and the counter electrode 2 with respect to the non-aqueous electrolyte solution 4. It is installed so as to be located between the counter electrodes 2.
(Comparative Example 1)
A lithium secondary battery was assembled in the same manner as in Example 1 except that a non-aqueous electrolytic solution to which hydrogen peroxide was not added was used.
(Evaluation of deposits by electrodeposition test)
Each of the lithium secondary batteries of Example 1 and Comparative Example 1 was used, and the charge / discharge device 7 for electrode performance evaluation was used to set the current density of the charging current to 1 mA / cm ² and the charge amount of charging to 5 C / cm ² . An electrodeposition test was conducted in which lithium was eluted from 2 and lithium was deposited on the surface of the working electrode 1. After the electrodeposition test, the surface of the working electrode 1 and the counter electrode 2 of each lithium secondary battery was observed using a scanning electron microscope (SEM).
[0020]
FIG. 2 shows four SEM photographs showing the surface portions of the working electrode and the counter electrode of Example 1 and Comparative Example 1 after the electrodeposition test. In FIG. 2, the upper left SEM photograph shows the surface state of the working electrode of Example 1, and the lower left SEM photograph shows the surface state of the counter electrode of Example 1. The SEM photograph at the upper right shows the surface state of the working electrode of Comparative Example 1, and the SEM photograph at the lower right shows the surface state of the counter electrode of Comparative Example 1.
[0021]
From the SEM photograph at the upper left of FIG. 2, it can be seen that fine granular precipitates are uniformly deposited on the surface of the working electrode of Example 1, and no dendrite is generated. On the other hand, dendrites are generated on the surface of the working electrode of Comparative Example 1 from the SEM photograph in the upper right of FIG.
Moreover, it can be seen from the SEM photograph at the lower left in FIG. 2 that the surface of the counter electrode of Example 1 is extremely flat. This indicates that elution of lithium occurs uniformly. On the other hand, it can be seen from the SEM photograph at the lower right in FIG. 2 that holes are scattered on the surface of the counter electrode of Comparative Example 1. This hole was formed due to non-uniform elution of lithium.
[0022]
From these facts, it can be seen that the addition of hydrogen peroxide to the non-aqueous electrolyte can suppress the formation of dendrite on the negative electrode surface.
(Evaluation of cycle characteristics by electrodeposition test)
The lithium secondary batteries of Example 1 and Comparative Example 1 were used, respectively, and the charge / discharge current density was set to 1 mA / cm ² by the charge / discharge device 7 for evaluating electrode performance, and the charge amount per cycle was 5 C / cm ^2. As described above, lithium is eluted from the counter electrode 2 to deposit lithium on the surface of the working electrode 1, and then lithium is eluted from the working electrode 1 to deposit lithium on the surface of the counter electrode 2. The cycle was repeated 8 times. After this electrodeposition test, the surface of the working electrode 1 and the counter electrode 2 of each lithium secondary battery was observed using an SEM.
[0023]
FIG. 3 shows an SEM photograph showing the surface portion of the working electrode of Example 1 and Comparative Example 1 after this electrodeposition test. In FIG. 3, the left SEM photograph shows the surface state of the working electrode of Example 1, and the right SEM photograph shows the surface state of the counter electrode of Comparative Example 1.
From the SEM photograph on the left side of FIG. 3, it can be seen that no dendrite residue is observed on the surface of the working electrode of the lithium secondary battery of Example 1. From this, it can be seen that in the lithium secondary battery of Example 1, even when lithium deposition and elution are repeated, electrochemically inactive so-called dead lithium is not formed on the negative electrode surface.
[0024]
On the other hand, from the SEM photograph on the right side of FIG. 3, a dendrite-like residue was observed on the surface of the working electrode of the lithium secondary battery of Comparative Example 1, and it died due to repeated lithium deposition and elution. It can be seen that lithium is formed.
Therefore, by adding hydrogen peroxide to the non-aqueous electrolyte, the lithium secondary battery has excellent cycle characteristics, with low lithium consumption and high charge / discharge efficiency even when lithium deposition and elution are repeated. It can be seen that
[0025]
【The invention's effect】
Since the lithium secondary battery of the present invention has excellent cycle characteristics and high safety, the lithium secondary battery can be used many times based on high reliability as a battery for portable electronic devices or electric vehicles.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of a lithium secondary battery of Example 1. FIG.
FIG. 2 is four SEM photographs showing the surface portions of the working electrode and the counter electrode of Example 1 and Comparative Example 1 in the evaluation of precipitates by an electrodeposition test.
FIG. 3 is two SEM photographs showing the surface portion of the working electrode of Example 1 and Comparative Example 1 in the evaluation of cycle characteristics by an electrodeposition test.
[Explanation of symbols]
1: Working electrode 2: Counter electrode 4: Non-aqueous electrolyte 10: Lithium secondary battery

Claims (1)

A secondary battery having a non-aqueous electrolyte and having a negative electrode made of lithium metal or a lithium alloy, wherein the non-aqueous electrolyte contains 10 ⁻⁴ mol / l or more and 10 ⁻¹ mol of hydrogen peroxide capable of oxidizing lithium. Lithium secondary battery characterized by containing / l or less .

Images