JP4798507B2 - Method for producing single-crystal LiMn2O4 nanowire and high-rate Li-ion battery using single-crystal LiMn2O4 nanowire - Google Patents

Description

The present invention relates to a high-rate Li-ion battery using the manufacturing method and a single crystal LiMn ₂ O ₄ nanowires monocrystalline LiMn ₂ O ₄ nanowires.

In recent years, energy and environmental issues have been actively taken up, and it has been necessary to develop clean energy devices as a means of solving these problems. The demand is very high. As an energy source for electric vehicles, there is a high possibility that a Li-ion battery has. This is because Li-ion batteries have a high energy density.
However, Li-ion batteries have the disadvantage of low power density. In order to solve this problem, many inventors have studied Li storage devices for electric vehicles.

The Li ion battery requires a positive electrode and a negative electrode, but in the present invention, attention was paid to the positive electrode material. Current LiCoO ₂ is expected to be difficult for future automotive applications (necessary in large quantities) given the high cost of Co. On the other hand, since Mn is an inexpensive material, there is no problem regarding its spread. Mn compounds such as LiMnO ₂ having a layered structure have been studied as positive electrode materials. However, since they are unstable with respect to the electrochemical cycle, capacity degradation due to repeated charge and discharge is a problem. In order to solve this problem, stable LiMn ₂ O ₄ having a spinel structure has been studied.
In addition, there are four problems to be solved in order to develop a high-rate Li storage device that can rapidly charge and discharge.
1) Reduction of particle size to reduce the diffusion length of Li in the active material. 2) A decrease in current density during a rapid charge / discharge process. 3) Improvement of cycle characteristics in a rapid charge / discharge process. 4) There are four problems of improving the electron conductivity of the electrode material.
In other words, a high-rate device cannot be realized without nanostructure control.

So far, there has been no report of good high rate characteristics using LiMn ₂ O ₄ . This is because, first of all, even if nanoparticles can be produced, they can be easily agglomerated into large secondary particles when used as an electrode material and cannot exhibit the characteristics of the original nanoparticles. Second, in order to have a stable charge / discharge potential, it is necessary to produce LiMn ₂ O ₄ having high crystallinity, and heat treatment must be performed at a high temperature. This is probably because it is difficult to suppress the grain growth by this high-temperature heat treatment, and the nanostructure required by the high-rate Li-ion battery could not be obtained.
Considering the suppression of grain growth during agglomeration and high-temperature heat treatment, it is inappropriate to produce nanoparticles for producing high-rate Li-ion battery electrodes. It is believed that there is. This is because the non-woven fabric formed of nanowires is already immobilized by wires and does not aggregate while having a porous structure. In addition, the number of contacts between the nanowires is very small, and the structure of the nanowires is maintained without grain growth even by heat treatment at a high temperature. Therefore, it is possible to have a nanostructure necessary for the electrode structure of a high-rate Li-ion battery, and if it is a single crystal wire structure, the electrical resistance due to the grain boundary is reduced, and the structure is more suitable as a higher-rate Li-ion battery electrode. I understand that there is.

If an electrode material having good crystallinity and a nanostructure can be produced, it is expected that even at a high rate, a battery characteristic having a flat plateau in the discharge curve is expected. Among the characteristics superior to the capacitor of a battery, one has a large capacity, and furthermore, a stable voltage can be supplied. The high-rate Li storage devices reported so far do not show a flat plateau in most reports and are called supercapacitors rather than batteries, and are similar to discharge curve capacitors and can supply a stable voltage. could not.
Currently, graphite electrodes used as negative electrode materials are mixed with carbon powder together with graphite powder to improve the filling rate and volume of the electrode active material, thereby contributing to improved cycle characteristics. .
The present inventor has already proposed a high-rate battery electrode using sodium manganate (Na _0.44 MnO ₂ ) single crystal nanowire (Japanese Patent Application No. 2007-067835).
Japanese Patent Application No. 2007-067875

In the present invention, a single crystal nanowire made of a material having a cubic crystal structure is manufactured. A single-crystal spinel-type LiMn ₂ O ₄ nanowire is fabricated, and as a Li-ion battery cathode material, it exhibits a large capacity even from low to high power density, and has stable cycling characteristics and a stable plateau. Provide material.

That is, in the present invention, Mn ₃ O ₄ and 1 to 20 M sodium hydroxide aqueous solution are reacted at 1 to 500 atm, 180 to 250 ° C. for 6 to 240 hours, and the reaction product is washed with water and dried. Crystalline sodium manganate (Na _0.44 MnO ₂ ) nanowires, and the single crystal sodium manganate (Na _0.44 MnO ₂ ) nanowires were washed with water, dried, and the single crystal sodium manganate and excess LiNO ₃ / LiCl is reacted at 400 to 500 ° C., then washed with water, dried, and heat treated at 600 to 1000 ° C. to produce a single crystal LiM ₂ O ₄ nanowire.
In the method for producing nanowires of the present invention, washing with water can be performed with ion-exchanged water.

The single crystal LiMn ₂ O ₄ nanowire obtained by the production method of the present invention is not only simple in production method, but the electrode using this single crystal LiMn ₂ O ₄ nanowire exhibits a large capacity and is stable. Li-ion battery positive electrode materials having cycle characteristics and stable plateaus can be provided.

A commercial item can be used for Mn ₃ O ₄ used in the present invention.
The sodium hydroxide aqueous solution used in the present invention needs to use 1 to 20M.
Below 1M, the reaction is slow, and when above 20M, good quality single crystal sodium manganate (Na _0.44 MnO ₂ ) nanowires cannot be obtained.
In the present invention, the reaction pressure is suitably 1 to 400 atmospheres.
Below 1 atm, the reaction is slow, and when it is above 500 atm, good quality single crystal sodium manganate (Na _0.44 MnO ₂ ) nanowires cannot be obtained.
In the present invention, the reaction temperature is suitably 180 to 250 ° C.
Below 180 ° C, the reaction is slow, and above 250 ° C, good quality single crystal sodium manganate (Na _0.44 MnO ₂ ) nanowires cannot be obtained.
Furthermore, in the present invention, the reaction time needs 6 hours to 240 hours.
In the present invention, the single-crystal sodium manganate (Na _0.44 MnO ₂ ) nanowire and the single-crystal LiMn ₂ O ₄ nanowire may be washed with water as long as they have good purity. desirable.

Furthermore, in the present invention, when the single crystal sodium manganate (Na _0.44 MnO ₂ ) nanowire is reacted with excess LiNO ₃ / LiCl, a temperature of 350 to 600 ° C. is suitable, but more preferably 450 ° C. Degree is desirable.
Furthermore, in the present invention, the reaction product is washed with water, dried and heat-treated, and the temperature is preferably 600 to 1000 ° C. More preferably, a single crystal LiMn ₂ O ₄ nanowire having a good quality at about 800 ° C. is obtained.
Furthermore, in the present invention, well-known materials can be used as the conductive assistant, but a carbon material can be preferably used.
As the binder, a binder known in the industry can be used.
A single-crystal sodium manganate (Na _0.44 MnO ₂ ) nanowire, a conductive additive, and a binder are mixed and molded to form a high-rate electrode having an arbitrary shape.
Using such an electrode, a high-rate Li-ion battery can be produced.
The present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

(Manufacture of single crystal LiMn ₂ O ₄ nanowires)
Mn ₃ O ₄ was added to a 5M aqueous sodium hydroxide solution, placed in a Teflon (registered trademark) inner cylinder in a stainless steel sealed container, and subjected to a hydrothermal reaction at 205 ° C. for 4 days. The produced sodium manganate (Na _0.44 MnO ₂ ) single crystal nanowire was washed with ion-exchanged water and dried. This sodium manganate and excess LiNO ₃ / LiCl were reacted at 450 ° C. for 1 hour, then washed with ion-exchanged water and dried. This was heat-treated at 800 ° C. for 1 hour to obtain single crystal LiMn ₂ O ₄ nanowires.
(Manufacture of electrodes)
The produced LiMn ₂ O ₄ was mixed with carbon, which is a conductive additive, and then mixed with a binder, and pressed onto a SUS mesh current collector, which was used as an electrode.
(Manufacture of Li batteries)
Electrochemical evaluation was performed using metallic Li for the counter electrode and reference electrode, and an EC / DEC mixed solvent containing 1M LiClO ₄ for the electrolyte.
FIG. 1 shows the XRD of LiMn ₂ O ₄ produced.
It can be seen that spinel-type LiMn ₂ O ₄ that matches the JCPDS pattern was produced.

An electron microscope (SEM) photograph of LiMn ₂ O ₄ obtained in Example 1 is shown in FIG. From FIG. 2, it can be seen that the nanowire structure has a diameter of about several tens to 150 nm, and the aspect ratio is very large as 1000 or more. Moreover, it takes the form of a nonwoven fabric with nanowires, and has a structure in which aggregation is suppressed.
The electron microscope (TEM) photograph and electron beam diffraction of LiMn ₂ O ₄ obtained in Example 1 are shown in FIG.
From the electron microscope (TEM) photograph and electron beam diffraction of FIG. 3, it can be seen that the obtained nanowire is a single crystal.

Regarding the electrochemical characteristics of LiMn ₂ O ₄ obtained in Example 1, Honjo Chemical (Reference Example 1) LiMn ₂ O ₄ , and Mitsui Kinzoku (Reference Example 2) 5 wt% -Mg-doped LiMn ₂ O ₄ The cyclic voltammetry (0.1 mV / s) and charge / discharge curve (0.1 C, 1 C = 100 mA / g) at the cycle are shown in FIGS. 4 (a) and 4 (b). In cyclic voltammetry, only those using nanowires show sharp peaks at the two potentials of Li deinsertion, indicating the high crystallinity. Also, from the charge / discharge curve, when nanowires are used, as with the cyclic voltammetry results, the two-stage plateau is shown by the potential of the two types of Li insertion / desorption, and the capacity is the largest. The commercially available charging / discharging curve shows a gentle charging / discharging curve in which the two kinds of potentials are not constant and does not show a two-stage plateau.

FIG. 5 shows a second charging / discharging curve when charging / discharging at a high rate (50 C) using the electrode obtained in Example 1.
When nanowire is used, even at a high rate of 50C, the capacity exceeds 100mA / g, and the charge / discharge curve has a plateau at the high rate while maintaining the low rate capacity shown in FIG. Is shown. The commercial capacity is greatly reduced from the low rate capacity of FIG. 4, and the charge / discharge curve cannot show a plateau. Also in FIG. 6, it can be seen that the electrode using nanowires shows a flat plateau in the discharge curve under very high rates of 100C and 200C.

FIG. 7 shows the charge / discharge cycle characteristics of the nanowire and the commercially available sample using the electrode obtained in Example 1. Both show good cycle characteristics even when 100 cycles are performed at a high rate of 50C, but the capacity is the highest when nanowires are used. It can be seen that LiMn ₂ O ₄ nanowires have a large capacity and good cycle characteristics even at high rates.
Reported C-rate (1C = 100 mA / g) and capacity using nanowires obtained in Example 1 and electrodes obtained with commercial samples and using reported LiMn ₂ O ₄ nanoparticles The relationship is shown in FIG.
When nanowires are used, the capacity decrease with increasing rate is slight, and it can be seen that a large capacity is maintained even at a high rate. Commercial samples are found to decrease in capacity as the rate increases, and reported values using nanoparticles show a sharp decrease in capacity. This graph also shows the good characteristics of high-rate Li-ion batteries using nanowires.

The single crystal LiMn ₂ O ₄ nanowire obtained by the production method of the present invention is not only simple in production method, but the electrode using this single crystal LiMn ₂ O ₄ nanowire exhibits a large capacity and is stable. Since a Li-ion battery positive electrode material having cycle characteristics and a stable plateau can be provided, the utility value is extremely high in the industry.

XRD of LiMn ₂ O ₄ obtained in Example 1 Electron microscope (SEM) photograph of LiMn ₂ O ₄ obtained in Example 1 Electron microscope (TEM) photograph and electron diffraction pattern of LiMn ₂ O ₄ obtained in Example 1 Example 1 LiMn ₂ O ₄ obtained by, Honjo Chemical Co. (Example 1) LiMn ₂ O _4, Mitsui Mining & Smelting manufactured (Reference Example 2) two cycles of the obtained electrode with 5 wt% -Mg-doped LiMn ₂ O ₄ Eye cyclic voltammetry (0.1mV / s) Fig. 4 (a) and charge / discharge curve (0.1C, 1C = 100mA / g) Fig. 4 (b) Second charge / discharge curve when charging / discharging at high rate (50C) using the electrode obtained in Example 1 When the electrode obtained in Example 1 is used for charging / discharging at a low rate of 1 C to a high rate of high output (50 C (5 A / g), 100 (10 A / g), 200 C (20 A / g)) The second discharge curve Using the electrode obtained in Example 1, charge and discharge cycle characteristics of nanowire and a commercially available sample Reported C-rate (1C = 100 mA / g) and capacity using nanowires obtained in Example 1 and electrodes obtained with commercial samples and using reported LiMn ₂ O ₄ nanoparticles connection of

Claims (2)

Mn ₃ O ₄ and a 1-20M sodium hydroxide aqueous solution are reacted at 1-500 atm. At 180-250 ° C. for 6-240 hours, and the reaction product is washed with water and dried to obtain a single crystal sodium manganate (Na _0.44 MnO ₂ ) nanowires, and further, the single crystal sodium manganate (Na _0.44 MnO ₂ ) nanowires were washed with water and dried, and the single crystal sodium manganate and excess LiNO ₃ / LiCl were converted into 400 A method for producing a single crystal LiM ₂ O ₄ nanowire, characterized by reacting at ˜500 ° C., washing with water, drying, and heat-treating at 600 ° C. to 1000 ° C. The method for producing LiMn ₂ O ₄ nanowires according to claim 1, wherein washing with water is performed with ion-exchanged water.