JP2015221726A - Monolayer carbon nano-tube including phosphorus, anode of lithium and sodium ion secondary battery containing the same and production method of the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an anode of a lithium and sodium ion secondary battery which is formed by causing phosphorus not functioning as a secondary battery to be included in a monolayer carbon nano-tube.SOLUTION: A production method of an anode of a lithium and sodium ion secondary battery comprises opening the end part of carbon nano-tube by an oxidation treatment of cleaned carbon nano-tube, sealing the carbon nano-tube 11 and a phosphorus powder 13 in a glass tube 15 in a vacuum, arranging the glass tube 15 in an electric furnace 17 and heat-treating at 500-600 degrees for 3-10 h to introduce phosphorus into the carbon nano-tube 11 and removing phosphorus precipitated on the outer surface of the carbon nano-tube 11. The carbon nano-tube is optionally subjected to an annealing treatment before the oxidation treatment of cleaned carbon nano-tube.

Description

The present invention relates to a single-walled carbon nanotube encapsulating phosphorus, a negative electrode for lithium and sodium ion secondary batteries containing the same, and a method for producing the same.

  Conventionally, a carbon material used as a negative electrode of a lithium ion secondary battery has a problem that the amount of stored lithium ions is small. For this reason, research on metal-based negative electrodes and alloy-based negative electrodes such as silicon has been underway as high-capacity negative electrodes.

  However, since the lithium metal negative electrode has high reactivity and easily generates dendrite, safety has been a problem.

For the silicon negative electrode, low electrical conductivity and large volume change during charge / discharge were problems. Research has been conducted on the use of silicon nanowires, etc. in order to suppress electrode pulverization due to volume changes during charge and discharge, or to make composites with carbon materials to improve electrical conductivity. There is no improvement in efficiency loss.

On the other hand, the lithium ion secondary battery also has a resource problem. Lithium is a rare element, and its resources are concentrated in South America and its amount is small. As large electric devices such as electric vehicles become more widespread, demand for lithium is expected to increase rapidly. For this reason, a sodium ion secondary battery advantageous in terms of resources is expected.

However, the graphite used as the negative electrode of the lithium ion secondary battery cannot be used for the sodium ion secondary battery. This is because graphite does not form a lower order intercalation compound with sodium.

There is a report that hard carbon (non-graphitizable carbon) can be used as a negative electrode of a sodium ion secondary battery. However, the capacity of hard carbon can store sodium ions only about 300 mAh / g.

On the other hand, non-patent literature (Adv. Mater. 2007, 19, 2465-2468) reports on the use of black phosphorus as a negative electrode for sodium ion batteries. In this research, it is shown that black phosphorus having conductivity is used instead of red phosphorus, which is an insulator, to function as an electrode.

However, to produce black phosphorus, high temperature and high pressure are required such as applying a pressure of 200 GPa or more and 1.2 GPa or more. Moreover, since the pulverization by the volume change at the time of charging / discharging cannot be suppressed by using black phosphorus, the capacity | capacitance fall by repeating a charging / discharging cycle is not suppressed. Furthermore, although black phosphorus has conductivity, it has a problem that a high output cannot be produced because electric conductivity is poor.

It is conceivable to use a carbon material as a conductive auxiliary agent as a method for compensating for low electrical conductivity, which is a problem when using phosphorus as a negative electrode active material. However, simply mixing carbon black cannot solve the problem of capacity reduction associated with charge / discharge cycles.

Since phosphorus electrochemically forms a metal phosphide with lithium and sodium, it can be used as a negative electrode for conventional lithium ion secondary batteries and sodium ion secondary batteries.

C. M. Park et al., Adv. Mater. 2007, 19, 2465-2468

The current production of the phosphor negative electrode requires a high pressure of 1.2 GPa or more and a high temperature of 200 degrees or more at the same time, so that the amount of energy used for the production of the electrode is large. Therefore, since it is not possible to use red phosphorus or white phosphorus as an electrode as it is, a technique for producing a phosphorus negative electrode more easily is desired.

  Phosphorus and its allotropes, white phosphorus, red phosphorus, and black phosphorus, have poor conductivity, and therefore have high resistance during charging and discharging. Poor conductivity causes overvoltage during charging and discharging, and that much energy is lost.

  Electrode pulverization due to volume changes that occur during charging and discharging is known to deteriorate the cycle characteristics of the battery, and electrode degradation is also reported when black phosphorus is used.

The negative electrode material for sodium ion batteries has a short history of research, and no promising negative electrode material has been found. Hard carbon functions as a negative electrode for the time being, but has a drawback that the capacity (amount capable of storing sodium ions) is small and the discharge potential is not constant.

As a negative electrode of a lithium ion secondary battery, a graphite-based material has been used for many years. However, a material having a higher capacity is desired for a large-sized application such as an electric vehicle. Si or the like is expected as a high-capacity material, but there is a drawback in that the volume change during charging / discharging is large and the capacity deterioration accompanying charging / discharging is large.

The present inventors have proposed a method of effectively encapsulating phosphorus in single-walled carbon nanotubes using red phosphorus, which is non-toxic and stable in the air, and a single-walled carbon nanotube encapsulating phosphorus in a lithium ion secondary battery. And it was confirmed that it functions as a high-capacity negative electrode material when used as an electrode of a sodium ion secondary battery. Furthermore, when single-walled carbon nanotubes encapsulating phosphorus are discharged against lithium and sodium metal, an average discharge voltage of 0.59 V (vs. Li ⁺ / Li) for lithium and 0.18 V (vs. ^Since a discharge voltage of ⁺ / Na) was obtained, it was found that both capacity and potential were suitable as a negative electrode material.

 An object of the present invention is to provide a production method in which phosphorus that does not originally function as a secondary battery is encapsulated in a single-walled carbon nanotube, and a sodium ion secondary battery negative electrode and a lithium ion battery negative electrode using the same.

  According to the single-walled carbon nanotube encapsulating phosphorus according to the present invention, the negative electrode of the lithium and sodium ion secondary battery including the same, and the manufacturing method thereof, the capacity is three times or more that of hard carbon and the discharge potential is also constant The effect which is.

In addition, the volume change of phosphorus when Li is stored is smaller than that of Si, and by encapsulating in carbon nanotubes, even when pulverization occurs during charge and discharge, phosphorus can be retained in the electrode and the capacity can be prevented from decreasing.

FIG. 1 is a schematic view showing a process for producing single-walled carbon nanotubes containing phosphorus according to the present invention, wherein (a) shows a first process and (b) shows a second process. FIG. 2 is a TEM image of carbon nanotubes doped with phosphorus. Fig. 3 is a mapping photo image of carbon. FIG. 4 is a mapping photograph of phosphorus. FIG. 5 is a graph showing charge / discharge characteristics of lithium ions of carbon nanotubes doped with phosphorus. FIG. 6 is a graph showing the charge / discharge characteristics of sodium ions. FIG. 7 is a graph showing the frequency characteristics of the synthesized phosphorus-encapsulated carbon nanotubes and non-phosphorus-encapsulated hollow carbon nanotubes. FIG. 8 is a graph showing charge / discharge characteristics of a sample in which phosphorus and carbon black are mixed, in which (left) shows a lithium ion charge / discharge curve and (right) shows a sodium ion discharge curve. FIG. 9 is a schematic view showing a lithium or sodium ion secondary battery negative electrode using single-walled carbon nanotubes containing phosphorus.

A method for producing single-walled carbon nanotubes containing phosphorus according to the present invention will be described below.

(Synthesis)
Single-walled carbon nanotubes 11 having an average diameter of about 1.5 nm were used as the host material. In this example, the single-walled carbon nanotube 11 is synthesized by an arc discharge method. If the average diameter is smaller than about 1 nm, it is difficult to introduce phosphorus, and if it is larger than about 2 nm, it is not suitable because it becomes the same as bulk phosphorus.

At this time, carbon nanotubes may contain impurities and deposits such as metal nanoparticle catalysts due to their production, so removing them allows more efficient phosphorus inclusion and improved negative electrode performance. Can contribute.

The method for purifying the carbon nanotube is not particularly limited, and for example, cleaning using an acid (eg, hydrochloric acid, nitric acid, sulfuric acid, hydrogen peroxide, etc.) may be used. Note that after the cleaning, annealing may be performed at a high temperature (for example, 1000 to 2000 degrees), and this annealing is preferably performed under vacuum. After the annealing treatment, an oxidation treatment of about 500 degrees in air is performed to open the end of the tube so that phosphorus can be included.

When the annealing treatment is performed, if it is lower than 1000 degrees, improvement in crystallinity cannot be expected, and if it is higher than 2000 degrees, fusion of nanotubes occurs, which is not suitable.

Further, when the oxidation treatment is performed, if it is lower than 400 degrees, it is difficult to open ends, and if it is higher than 600 degrees, the disappearance of the nanotubes is large, which is inappropriate.

  Next, the carbon nanotube 11 and the phosphorus powder 13 subjected to the open end treatment are sealed in a glass tube 15 under vacuum. At this time, the degree of vacuum is desirably 1 Pa or less.

Next, the entire glass tube 15 is inserted and placed in an electric furnace 17 and heat-treated at 500 to 600 degrees for 3 to 10 hours to introduce phosphorus into the carbon nanotubes 11 (see FIG. 1A).

  If the temperature in the electric furnace 17 is lower than 500 degrees, it is difficult to introduce phosphorus, and if it is higher than 600 degrees, a side reaction occurs, which is not suitable.

  Moreover, if the flammable time in the electric furnace 17 is shorter than 3 hours, the amount of phosphorus introduced is small, and even if it is longer than 10 hours, it is not possible to introduce more phosphorus, which is not suitable.

Next, phosphorus is deposited not only on the inside of the carbon nanotubes 11 but also on the outer surface by this heat treatment, which is removed.

At this time, after the position of the glass tube 15 is shifted and the end of the carbon nanotube is taken out of the electric furnace 17, the phosphorus-containing carbon nanotube in the electric furnace 17 is heated at 200 degrees for 1 hour. At this temperature, phosphorus on the outer surface of the carbon nanotube 11 sublimates and precipitates at the end of the glass tube 15 outside the electric furnace 17 having a low temperature (see FIG. 1B).

The reason why the phosphorus-encapsulated carbon nanotubes are heated at 200 degrees for 1 hour is to remove phosphorus physically adsorbed on the outer surface of the nanotubes.

Thereafter, the glass tube 15 is broken, and the phosphorus-containing carbon nanotubes are taken out.

(Product evaluation)
In order to evaluate the structure of phosphorus-encapsulated carbon nanotubes, transmission electron microscope observations and Raman scattering experiments were performed.

It was confirmed by observation with a transmission electron microscope that there was no change in the tube form even when the encapsulation treatment was performed. Further, from Raman scattering, G-band and 2D-band peculiar to carbon nanotubes are observed even after the encapsulation treatment, and both of them show a high wave number shift after the encapsulation treatment, which is the effect of the encapsulated phosphorus (Fig. 7).

On the other hand, when the distribution of carbon and phosphorus was examined by STEM observation, both spatially showed the same distribution, and it was confirmed that phosphorus was introduced into the carbon nanotube.

(Negative electrode performance evaluation)
A test cell is constructed with phosphorus-encapsulated carbon nanotubes as the working electrode and metallic lithium or metallic sodium as the counter electrode (see FIG. 9).

The electrolyte used was a 1 mol / l lithium perchlorate or a mixture of ethylene carbonate in which 0.5 mol / l sodium perchlorate was dissolved and diethyl carbonate in a volume ratio of 1: 1.

As a result, a reversible capacity of about 1600 mAh / g as a lithium ion battery negative electrode and a reversible capacity of about 1200 mAh / g as a sodium ion battery negative electrode were observed (see FIGS. 5 and 6). This is in contrast to the fact that almost no reversible capacity is observed in a sample in which phosphorus is simply mixed with a conductive auxiliary such as carbon black (see FIG. 8).

The higher the concentration of lithium perchlorate and sodium perchlorate, the larger the number of ions, which is desirable, and 1 mol / l and 0.5 mol / l, which are close to the respective saturated concentrations.

The reason why the volume ratio of ethylene carbonate to diethyl carbonate is 1: 1 is to reduce the viscosity and improve the ionic conductivity.

  According to the single-walled carbon nanotube encapsulating phosphorus according to the present invention, the negative electrode of the lithium and sodium ion secondary battery including the same, and the manufacturing method thereof, phosphorus can be effectively used.

11 Carbon nanotube 13 Phosphorus powder 15 Glass tube 17 Electric furnace

Claims (6)

By oxidizing the washed carbon nanotubes, the ends of the carbon nanotubes are opened,
Next, the carbon nanotube 11 and phosphorus powder 13 are sealed in a glass tube 15 under vacuum,
Next, this glass tube 15 is placed in an electric furnace 17, heat treated at 500 to 600 degrees for 3 to 10 hours, and phosphorus is introduced into the carbon nanotubes 11;
Next, a method for producing single-walled carbon nanotubes containing phosphorus, wherein phosphorus deposited on the outer surface of carbon nanotubes 11 is removed. 2. The method for producing single-walled carbon nanotubes containing phosphorus according to claim 1, wherein annealing is performed before the oxidized carbon nanotubes are oxidized. The phosphorus-encapsulated nanotube according to claim 1 or 2 as a working electrode, metallic lithium or metallic sodium as a counter electrode,
As the electrolytic solution, a 1 mol / l lithium perchlorate or a mixed solution of ethylene carbonate in which 0.5 mol / l sodium perchlorate was dissolved and diethyl carbonate in a volume ratio of 1: 1 was used. A method for producing a negative electrode for a lithium and sodium ion secondary battery. By oxidizing the cleaned carbon nanotubes, the ends of the carbon nanotubes are opened, and then the carbon nanotubes 11 and phosphorus powder 13 are sealed in a glass tube 15 under vacuum, and then the glass tube 15 is sealed in an electric furnace. 17 and heat-treated at 500 to 600 degrees for 3 to 10 hours to introduce phosphorus into the carbon nanotube 11 and then remove phosphorus deposited on the outer surface of the carbon nanotube 11 to enclose a single layer containing phosphorus carbon nanotube. 5. The single-walled carbon nanotube containing phosphorus according to claim 4, wherein the cleaned carbon nanotube is subjected to an annealing treatment before the oxidation treatment. The phosphorus-encapsulated nanotube according to claim 4 or 5 is used as a working electrode, metallic lithium or metallic sodium is used as a counter electrode, and 1 mol / l lithium perchlorate or 0.5 mol / l sodium perchlorate is dissolved as an electrolyte. A negative electrode for a lithium and sodium ion secondary battery, wherein a mixed liquid having a volume ratio of 1: 1 to ethylene carbonate and diethyl carbonate is used.