JP5679260B2 - Composite composed of sulfur and conductive polymer - Google Patents

Description

  The present invention relates to a composite composed of fibrous sulfur and a conductive polymer existing so as to surround it, and is useful as a positive electrode active material of a battery.

In recent years, with the rapid spread of portable electronic devices and electric vehicles, secondary batteries that can be repeatedly charged and discharged with high capacity are required and are actively developed. Among them, lithium batteries are particularly attracting attention because they are expected to be lightweight and have high output. At present, as lithium batteries, there are many cases where LiCoO ₂ or LiMn ₂ O ₄ is used for the positive electrode and carbon or metallic lithium is used for the negative electrode, but in the case of the negative electrode carbon, the electric capacity is 300-370 mAh / g, lithium In this case, the electric capacity of LiCoO ₂ and LiMn ₂ O ₄ of the positive electrode is about 110-140 mAh / g, whereas it is 3830 mAh / g, and development of a positive electrode material is desired.

On the other hand, sulfur has attracted attention as a positive electrode of a high energy density battery. Sulfur has a theoretical energy density of 2600 Wh / g and a theoretically high capacity of 1672 mAh / g, assuming complete reaction with lithium up to Li ₂ S. Furthermore, sulfur has the advantage of being inexpensive because it has low toxicity and is rich in resources.

  However, since sulfur is poor in reactivity and is an insulator, in order to be used for the positive electrode active material, it is necessary to increase the activity of sulfur and to impart conductivity. Therefore, in order to compensate for these drawbacks, many developments have been made. For example, as a secondary battery using metallic sodium as a negative electrode and sulfur as a positive electrode, a proposal to increase the operating temperature to 300 ° C. or higher in order to increase the activity of sulfur. Has been made. As a sodium / sulfur battery for improving the operation at such a high temperature, a mixture of 50 to 70% by weight of sulfur, 15 to 30% by weight of carbon, and 15 to 20% by weight of polyethylene oxide based on the weight of the whole mixture. As a positive electrode, a negative electrode as sodium or sodium-containing carbon or sodium oxide, and a glymid solution containing a sodium salt as an electrolyte, a sodium / sulfur battery operating at room temperature has been proposed (Patent Document 1). ing.

  In addition, a proposal (Patent Document 2) is proposed in which a composite in which sulfur fine powder and carbon fine powder are closely integrated by a mechanochemical fusion method is used as a positive electrode.

  However, a battery that operates at a high temperature of 300 ° C. has inconveniences such as an increase in size and lack of stability, and in a positive electrode that mixes sulfur and carbon, the mixing ratio of impurities other than sulfur increases. In addition, the sulfur compound is eluted in the electrolytic solution due to repeated charging and discharging, and the output is gradually reduced. Therefore, development of a positive electrode material that functions stably for a long time at a high energy density using sulfur as a positive electrode active material has been expected.

Special table 2007-522633 Japanese Patent Laid-Open No. 2006-09885

  The present invention develops and proposes a cathode material containing sulfur that is more efficient and highly stable in view of the above-described problems when sulfur is used as a cathode active material.

  A first aspect of the present invention is a composite substantially consisting of sulfur and a conductive polymer, characterized in that a monomer that becomes a conductive polymer is adsorbed on fibrous sulfur and then polymerized.

  Moreover, the 2nd aspect of this invention is a composite_body | complex substantially consisting of sulfur and a conductive polymer as described in the 1st aspect characterized by including 5-30 weight% of said conductive polymers.

  Further, according to a third aspect of the present invention, the fibrous sulfur is a non-woven cloth having an average diameter of 50 μm or less, preferably 10 to 30 μm, and is substantially electrically conductive with sulfur according to the first aspect or the second aspect. It is a composite made of a functional polymer.

  Further, according to a fourth aspect of the present invention, substantially the sulfur and the conductive material according to any one of the first to third aspects, wherein the monomer to be a conductive polymer is pyrrole and the conductive polymer is polypyrrole. It is a composite made of a polymer.

  According to a fifth aspect of the present invention, the sulfur and the conductive material according to any one of the first to fourth aspects are characterized in that the fibrous sulfur is produced by a melt electrospinning method. It is a composite made of a functional polymer.

  The present invention is based on the new knowledge that sulfur can be made into a fiber, and can be made into a fiber having a nano-order to micron-order diameter by an electrospinning technique, after adsorbing a monomer serving as a conductive polymer to the fiber. By covering the fiber surface mainly with a thin conductive polymer film by polymerization, it is possible to promote the activity of sulfur, which is an insulator, to provide conductivity, and to provide a positive electrode material for a battery that provides high-density energy. It can be.

  Further, in the present invention, since a conductive film is formed on the surface of the fiber, the outflow of the sulfur compound to the electrolytic solution is suppressed, so that the output is reduced and the voltage is lowered even with repeated charge and discharge. There are few etc. and it works stably.

1 is a schematic view of a sulfur melt electrospinning apparatus. FIG. SEM photograph of sulfur fiber (a) extruded while applying voltage and sulfur fiber (b) by melt electrospinning. The SEM photograph which shows an example of the composite_body | complex which consists of sulfur which coat | covered fibrous sulfur with the conductive polymer by this invention, and a conductive polymer. An example of the measurement result by the cyclic voltammetry in the composite_body | complex which consists of sulfur of this invention, and a conductive polymer. XRD spectrum of sulfur and its fibers An example of the measurement result by the cyclic voltammetry in the composite_body | complex which consists of sulfur of this invention, and a conductive polymer. An example of the measurement result by the cyclic voltammetry in the composite_body | complex which consists of sulfur of this invention, and a conductive polymer. The measurement result by the cyclic voltammetry in sulfur fiber.

  The point of the present invention is that it has been found that sulfur can be formed into a fibrous form, particularly an ultrafine fiber form. Conventionally, an attempt to increase the surface area of sulfur or a sulfur compound to make it an active electrode material is generally to atomize sulfur, and a method of joining them with carbon which is a conductive material has been recommended. In this case, the amount of carbon covering the entire surface of the sulfur particles is increased, and about 50% of carbon is mixed for a practical positive electrode. However, in the present invention, since it is made into a thin fiber shape, it is only necessary to cover the side direction, so the amount of the conductive material for obtaining the same effect can be reduced. For this reason, the sulfur ratio in the composite as the positive electrode active material can be increased.

Furthermore, in the present invention, since the monomer that becomes the conductive polymer is adsorbed on the fibrous sulfur, it can be uniformly attached to the surface of the sulfur or the fissures of the fiber, and the layer is extremely thin. By polymerizing this, a thin conductive polymer film can be formed. In this way, by forming a thin film, it is easy for the electrolytic solution to penetrate, and elution of the sulfur compound represented by Li _{2 Sn} (where n is the number of sulfur atoms, generally 8 or less) into the electrolytic solution. Has the advantage of being suppressed.

In the present invention, the shape of fibrous sulfur is not particularly limited. In general, the smaller the fiber diameter, the larger the surface area and the electrical conductivity is 5.0 × 10 ⁻¹⁶ Ω ^−1.
・ As low as m ⁻¹ , the reactivity of sulfur, which is an insulator, and the mobility of electrons are ensured, but the strength as a fiber is reduced and handling becomes inconvenient. In general, the diameter of the fiber is preferably 50 μm or less on average, and is 10 μm or more, preferably about 20 to 30 μm on average in terms of handling and processing.

  Further, the length of the fiber is not particularly limited. Generally, it is often used as a non-woven mat, and is 1 to 30 cm, preferably 20 to 30 cm.

  Although the manufacturing method of these fibers is not limited at all, it can generally be melted and extruded from a nozzle. That is, the melting point of sulfur is 112.8 ° C. (α sulfur) to 119.6 ° C. (γ sulfur), but the viscosity increases up to 195 ° C. and decreases again at higher temperatures. Therefore, melt spinning can be performed at 180 to 250 ° C., preferably about 190 to 220 ° C. One preferred spinning method is the electrospinning method.

  An outline of the melt electrospinning apparatus is shown in FIG. In the figure, when sulfur is charged in the syringe and heated, and the voltage exceeds the threshold value due to the voltage applied between the needle and the collector (current collector), the repulsive force of the charge causes the surface of the molten sulfur A charged jet is generated by overcoming the tension, and the jet stretches in the electric field to form very thin fibers, which are deposited on the collector. As conditions in the melt electrospinning method, when the melt electrospinning temperature is usually 180 to 250 ° C., if the distance between the needle and the collector is excessively short, electric discharge occurs between the needle and the collector and spinning cannot be performed. If the length is excessively long, spinning may not be possible because the electrostatic attraction force that pulls the fiber becomes small. Therefore, the distance is preferably 4 cm to 10 cm. In addition, when the applied voltage is excessively low, spinning may not be possible because the electrostatic attractive force is small, and when it is excessively high, discharge may occur between the needle and the collector and spinning may not be possible. Therefore, the applied voltage is preferably 8 kV to 10 kV. It should be noted that the ease of occurrence of discharge varies with changes in ambient conditions (mainly humidity), and conditions with a humidity as low as 20% or less are preferred. In this case, spinning is possible under conditions of a humidity of about 10 to 50%.

  Thus, sulfur fibers having a diameter of nanometer to micron size are obtained. In this case, the average diameter of the fiber can be almost determined by the viscosity of the molten sulfur, the applied voltage, and the distance between the needle and the collector. Therefore, the diameter of fibrous sulfur can be determined by trial and error of several degrees.

  FIG. 2 shows SEM photographs of sulfur fiber (a) prepared by extrusion while applying voltage and sulfur fiber (b) obtained by melt electrospinning (6 cm between needle and collector, applied voltage 10 kV, melting temperature 200 ° C.).

  In general, when a positive electrode of a battery is used, the melt spinning is performed on a current collector, and the current collector is a plate-like, net-like, or uneven metal, for example, copper, platinum, iron, nickel, etc. Carbon may also be used. In consideration of the contact between the positive electrode active material and the current collector, a part of the conductive polymer described later may cover the current collector.

  Next, the fibrous sulfur is integrated with a conductive polymer. The conductive polymer first adsorbs the monomer to the fibrous sulfur and polymerizes the adsorbed monomer on the sulfur to form a thin film. Need to be formed.

  A well-known monomer can be used as a monomer for conductive polymers. For example, monomers that form polyparaphenylene, polypyrrole, polyaniline, polythiophene, and the like. Among these, pyrrole is particularly preferable from the viewpoint of ease of polymerization and the state of the obtained film.

  The method for adsorbing the monomer to fibrous sulfur is achieved by placing fibrous sulfur as it is or, in some cases, a non-woven fabric deposited on a current collector as it is in a monomer solution or monomer gas. Is done. In this case, the treatment may be performed in advance in a polymerization catalyst and the catalyst may be attached in advance, or the monomer may be adsorbed and then treated with the catalyst. In some cases, polymerization can be performed by ultraviolet irradiation or heating.

  In the case where the polymerization is performed by the catalyst treatment, it is preferable to adsorb the monomer in advance and then treat with the catalyst after the catalyst is adsorbed in advance because the amount of monomer polymerization can be controlled more easily.

  It may also be preferable to dope a substance that enhances the conductivity of the conductive polymer such as iodine after coating the fibrous sulfur with the conductive polymer or during the formation of the conductive polymer.

FIG. 3 shows an SEM photograph of a composite of substantially sulfur and a conductive polymer obtained by polymerizing pyrrole using iodine as a catalyst for fibrous sulfur. In the figure, (1 ') and (2') are based on the conditions shown in Table 1 below.

In the figure, (a) shows the pattern of the whole nonwoven fabric, and (b) shows the state of each one fiber. In addition, in 1 '(b), especially a fiber cut surface is shown. From this example, it can be seen that the conductive polymer mainly covers the sulfur surface and penetrates and polymerizes just before the interior.

FIG. 4 shows an example of results obtained by performing cyclic voltammetry (CV) measurement of the oxidation-reduction potential and the current amount in order to see the electrochemical performance obtained by the present invention. In FIG. 4, it is a figure which started from 3.5 volts and performed 1 to 4.5 volts for 10 cycles. In the first cycle, a slight peak due to the reduction reaction is observed at 2.0 and 2.8 volts, but after the fifth cycle, there is a slight shift in the reduction peak potential, but 2.5-3. A reduction peak is seen at a high potential of .5 volts. Further, the reduction current increases with the number of cycles, and the reduction current maintains a large value even at the 10th cycle. As for these factors, it is understood that the composite of the present invention is that the conductive polymer is closely added to the sulfur fiber as an insulator, the conductive path is established, and the progress of the sulfur redox reaction is promoted. Is done.
Examples are shown below.

(Manufacture of fibrous sulfur)
(1) Melt electrospinning apparatus A silicon cord heater (1. 5m) [Reciprocal Chemical Glass Manufacturing Co., Ltd. SKH-0151] was wound. A stainless steel plate (9 × 9 cm) was used as a collecting plate. A high voltage power supply [Matsusada Precision Co., Ltd.] was connected so that the stainless steel needle was positively charged and the collector collector was negatively charged when voltage was applied. The temperature of the cord heater was measured and controlled by a contact temperature sensor [Tasco Japan Co., Ltd. THI-303F] and a sensor-powered digital meter relay [Tasco Japan Co., Ltd. TAT-806A]. A schematic diagram of an apparatus used for melt electrospinning is shown in FIG.
(2) Preparation of fibrous sulfur As a preparation method, after adding sulfur powder to a glass syringe, the glass syringe was heated to 200 degreeC with the silicon cord heater. Thereafter, a voltage was applied to the stainless needle and the collecting plate to perform melt electrospinning of sulfur on the collecting plate. In order to search for optimum spinning conditions, spinning is performed by changing the applied voltage and the distance from the tip of the stainless needle to the current collector.

  In order to perform electrochemical measurement (CV), a platinum mesh (3 × 20 mm) was placed in a collecting plate shape as a current collector for forming an electrode. The spinning conditions are summarized in Table 2.

  The distance from the tip of the stainless needle to the collecting plate or current collector is expressed as the spinning distance.

FIG. 5 shows XRD spectra of the fiber and sulfur powder (a) immediately after melt electrospinning of sulfur (b), 2 days later (c), and 5 days later (d). In the XRD spectrum of the sulfur powder, the peak observed at 2θ = 22-23 ° disappeared in the sulfur fiber immediately after spinning, and a new peak appeared in the vicinity of 2θ = 20,24 ° that was not found in the sulfur powder. . Therefore, it can be seen that the sulfur fiber produced by the melt electrospinning method was spun with a crystal structure of polymer sulfur. However, in the XRD spectrum two days and five days after the production of the sulfur fiber, the peak near 2θ = 20,24 ° seen in the fiber immediately after spinning disappears again, and the same XRD spectrum as the sulfur powder is obtained. It was. For these reasons, the crystal structure of the fiber immediately after spinning is unstable, and changes to α sulfur, which is known as a stable crystal structure of elemental sulfur, over time.
(3) Coating of conductive polymer (polypyrrole) Polypyrrole (PPy) polymerization by Pyrolele (pyrrole; Py) contact after adsorption of iodine (iodine):
Using the sulfur fiber produced under Condition 3 in Table 2, coating with PPy was performed. Table 3 shows each condition. Sulfur melt-spun (Pt / S) on a Pt mesh (5 mm x 3 mm) (Pt / S) is placed in a glass cell containing iodine and degassed with Ar gas. ), Iodine was adsorbed to the fiber for a predetermined time (Pt / S / iodine). Thereafter, Pt / S / iodine was transferred into a glass cell containing Py, deaerated with Ar gas, and then contacted with Py at 50 ° C. (in an oil bath) for a predetermined time. After the polymerization, drying was performed under reduced pressure at 50 ° C. for 24 hours. Table 3 shows the amount of PPy produced / (S + PPy) amount (%).

PPy polymerization by iodine contact after Py adsorption:
Using the sulfur fiber produced under Condition 3 in Table 2, coating with PPy was performed. Table 4 shows each condition. Pt / S was allowed to stand in a glass cell containing Py, degassed with Ar gas, and then Py was adsorbed to the fiber for a predetermined time in a dark place at 50 ° C. (in an oil bath) (Pt / S / Py). . Thereafter, Pt / S / Py was transferred into a glass cell containing iodine, degassed with Ar gas, and adsorbed with iodine in a dark place at 50 ° C. (in an oil bath) for a predetermined time. After the polymerization, drying was performed under reduced pressure at 50 ° C. for 24 hours. The amount of PPy produced / (S + PPy) amount (%) is shown in Table 4.

(4) Electrochemical behavior Cyclic voltammetry (CV) measurement:
In order to investigate the electrochemical behavior of the electrode coated with the conductive polymer under the conditions shown in Table 5 below using the fibrous sulfur shown in 3 of Table 2, CV measurement was performed. For comparison, a sulfur electrode was prepared by depositing sulfur fibers on a platinum mesh and then drying under reduced pressure at 50 ° C. for 24 hours. This sulfur electrode is referred to as an Sf electrode. The electrochemical system [Hokuto Denko Co., Ltd. HZ-5000] was used for the measurement. Measurement was performed with a potential scanning range of 1.0 V to 4.5 V and a scanning speed of ¹ mVs ⁻¹ for all the electrodes, and the cell temperature was kept constant at 30 ° C. in a thermostatic chamber [Tokyo Science Instruments EYELA MG-2300]. Went.

  All these operations were performed in a glove box under an argon atmosphere.

The electrolyte is 1 mol LiPF ₆ / propylene carbonate-diethyl carbonate (volume ratio 1: 1). Table 5 shows the numbers of the CV measurement graphs.

Claims (5)

After adsorption of the monomer which becomes conductive polymer fibrous sulfur, complexes consisting of sulfur and conductive polymers you comprising polymerizing. Claim 1 sulfur and conductive composite comprising polymeric according, characterized in that it comprises a conductive polymer 5 to 30 wt%. Fibrous sulfur, the average is a diameter 50μm or less, the complex consisting of sulfur and a conductive polymer of claim 1 or 2, wherein forming a nonwoven fabric. Complex consisting of sulfur and the conductive polymer according to any of claims 1 to 3 conductive polymer is a polypyrrole. Complex fibrous sulfur consisting of sulfur and the conductive polymer according to any of claims 1 to 4, characterized in that it is produced by a melt electrospinning method.