JPH0457073B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a nonaqueous battery using an organic solvent, and particularly to a battery with improved discharge characteristics and low internal resistance that uses activated carbon fibers having a specific pore structure as a positive electrode. It is. Batteries not only have the ability to directly generate electricity and store energy, but also have mobility, and are highly efficient in the energy conversion process, so their characteristics have been reviewed and batteries have been developed to meet various needs. development is underway. One of the demands is particularly for the miniaturization of batteries built into small cordless devices, that is, the increase in energy density, and it is believed that the range of applications will be expanded if these batteries have the performance of secondary batteries. The two main types of small secondary batteries currently available on the market are nickel cadmium alkaline batteries and silver oxide batteries. However, nickel-cadmium alkaline batteries have a long cycle life but low energy density. Although silver oxide batteries have a high energy density, they have drawbacks such as a short cycle life and, since they use silver, they are expensive.
In an attempt to overcome these drawbacks, an organic electrolyte-based secondary battery has been developed that uses a carbon material such as pyrolytic graphite for the positive electrode, a light metal such as lithium for the negative electrode, and uses inorganic or organic salts as the solute. This secondary battery is attracting attention because it is cycle-safe, uses inexpensive materials, and has a relatively high energy density, but there are still many points that need to be improved. For example, the mobility of ions in a solvent is an order of magnitude lower than in an aqueous solution system, so the internal resistance is large, and the flatness of the discharge potential is low in mercury batteries as primary batteries.
It is inferior to silver oxide batteries, especially at low temperatures. In view of these circumstances, we have arrived at the present invention as a result of intensive research. The purpose of the present invention is to provide a high energy density secondary battery with good discharge characteristics and low internal resistance. It uses an aggregate of activated carbon fibers as described above, and preferably has the same volume and electrical resistance.
It is characterized by using an aggregate of activated carbon fibers of 5.0×10 ^-2 Ω·cm or less. By the way, as already mentioned, the main cause of the drawbacks of conventional secondary batteries is that the mobility of ions in the electrolyte is low and the mobility of ions is large in temperature dependence. Therefore, one method is to use an electrolyte composition that has low viscosity and low temperature dependence, and examples of this are being considered in other non-aqueous batteries as well. This of course applies to the secondary battery, but in addition, it is extremely important to pay attention to the porous nature of the carbon material used for the positive electrode, and to select it appropriately as it depends greatly on the pore characteristics. I found out. Among the pore characteristics, a secondary battery such as the one of the present invention cannot be obtained simply by increasing the surface area. In this secondary battery, for example, when lithium is used for the negative electrode and an organic electrolyte containing lithium perchlorate is used as the electrolyte, the battery reaction is as follows: activated carbon fiber at the positive electrode is abbreviated as ACF, and ACF+a (clo ₄ ^- )Charge---→ ←---Emission [(ACF ^+a ) (clo ₄ ) _a ^- ]+ae ^-At the negative electrode, it is considered as aLi ⁺ + ^ae -Charge---→ ←---Emission aLi, and the negative The ions move to the pore surface of the positive electrode porous carbon material, forming an electric double layer (some say that it is doped).
The slow movement of ions within the pores affects the internal resistance or discharge characteristics. The speed of ion movement within a pore depends on the pore diameter. So-called micropores with a diameter of 30 Å or less are effective in increasing the surface area (increasing battery capacity), but from the viewpoint of ion movement, the pore size is too small. The speed of ion movement within the micropore is only 1/10 to 1/1000 of the speed of ion movement in the bulk system. A so-called transitional bore with a diameter of 30 Å or more is particularly preferable from the viewpoint of ion movement. However, if the pore size becomes too large, the surface area cannot be increased, so the upper limit of the pore size is set at 300 Å. The pore volume with a pore diameter of 30-300 Å is 0.1-0.2 in conventional activated carbon fibers.
in the cc/g range. Even if activated carbon fibers having such characteristics are used in the positive electrode, a battery with good discharge characteristics, particularly at low temperatures, cannot be obtained. The activated carbon fiber used in the present invention has a pore diameter of 30~
The pore volume of 300Å is 0.30cc/g or more,
By using such a positive electrode material, a secondary battery with low internal resistance, low temperature dependence of internal resistance, and flat discharge potential can be obtained for the first time. Furthermore, such performance can be more effectively achieved by setting the pore volume of pores with a diameter of 30 to 300 Å to 0.30 cc/g or more. Activated carbon fibers having such a specific porous structure are produced, for example, by the following method. That is, carbon fibers having a surface area of 30 to 1200 m ² /g, a pore diameter of 30 to 300 Å, and a pore volume of 0.1 cc/g or less are coated with at least one compound selected from compounds of Group A of the Periodic Table and transition metals. It is produced by loading the compound and then subjecting it to an activation treatment. As the above-mentioned activation aid, compounds of Periodic Table Group A elements such as magnesium, calcium, and barium, or transition metal elements such as iron, cobalt, nickel, and manganese are used. Water-soluble salts such as magnesium chloride, magnesium acetate, calcium chloride, ferric chloride, cobalt chloride, nickel acetate, and manganese chloride are most easily used. The method for supporting the activation aid is to immerse the starting carbon fiber in an aqueous solution of the above compound, then dehydrate and dry it.
Alternatively, there is a method in which the aqueous solution is sprayed and then dried, but the method is not limited thereto. The amount of impregnated activation aid is 0.01 to 40% by weight in terms of metal element.
is preferred. In addition, reactivation treatment is carried out in oxidizing gas containing water vapor, carbon dioxide, etc. or in combustion gas at 650~
A method of heating to 1050℃ can be applied. The reason why pores with a pore diameter of 30 to 300 Å increase when an activation aid is used is that the reaction rate between the carbon around the aid attached to the wall of the micropore and the activation gas increases significantly, and the micropore size increases. This is thought to be due to the progress of expansion and merging. By selecting the starting carbonaceous fiber in this way, loading it with a specific compound, and performing a reactivation treatment, it became possible for the first time to make it have a pore volume of 30 to 300 Å and 0.30 cc/g or more. Ta. The method of improving the discharge characteristics and reducing the temperature dependence of the internal resistance has been described above, but on the other hand, in order to lower the level of the internal resistance itself, it is necessary to lower the electrical resistance itself of the activated carbon fiber. The electrical resistivity of activated carbon fibers generally belongs to the semiconductor region and is quite large. Furthermore, since the fibers are porous, they tend to become larger, and at the same time, they vary greatly due to differences in temperature and activation temperature during production. The electrical resistivity of activated carbon single fibers is around 10 ^-1 Ω・cm, which is higher than that of graphite fibers (10 ^-3 Ω・cm), and the temperature during manufacturing is usually within a range where the resistance varies widely. It is. It must be said that this makes it difficult to obtain a secondary battery with high reliability and low internal resistance. Electrical specific resistance of single fiber is 5x
By setting the resistance to 10 ^-2 Ω·cm or less, preferably 1×10 ^-2 Ω·cm or less, a secondary battery with less fluctuation and a small internal resistance can be obtained. In order to obtain activated carbon fibers having an electrical resistivity of 5×10 ^-2 Ω・cm or less, the activated carbon fibers having a specific porous structure as described above are further subjected to temperature relief of 950°C or higher. It can be done. Preferably
Processing in an inert gas at a temperature of 1000°C or higher is recommended. What should be noted here is that pores with a diameter of 30 Å or less tend to change in pore diameter by heat treatment within the temperature range, whereas pores with a diameter of 30 Å or more maintain their diameter. Therefore, by using activated carbon fibers having a pore size and a small electrical resistivity as in the present invention, the internal resistance is small, the temperature dependence of the internal resistance is small, and the discharge potential can be maintained evenly. For the first time, it became possible to make batteries. Any known aggregate form of activated carbon fibers used in the polarizable electrode can be used, including aggregates in the form of felt, woven fabric, knitted fabric, mixed paper, and the like. Further, although the activated carbon fiber aggregate substantially contains activated carbon fibers, other positive electrode constituent materials may be mixed therein. Next, the activated carbon fiber aggregate is placed opposite a light metal negative electrode via a thin porous separator, impregnated with an organic electrolyte containing a solute, and the secondary battery is assembled under anhydrous conditions. As the separator, an electrolyte-resistant porous sheet, for example, a nonwoven fabric or porous sheet made of polyethylene, polypropylene, Teflon, etc., or glass fiber paper can be used. The electrolyte may be any organic solvent capable of dissolving the electrolyte salts, but considering the dielectric constant, viscosity, and its temperature dependence, propylene carbonate,
1,2-dimethoxyethane, γ-butyrolactone, tetrahydrofuran, dimethylformamide, 1,3-dioxolane, acetonitrile, etc. alone or a mixture thereof can be used. Care must be taken to prevent impurities from entering the electrolyte from the viewpoint of battery life and self-discharge, but in particular, water content must be kept below several ppm. In addition to water, it is particularly necessary to avoid contamination with lipophilic solvents, oils, and the like. The electrolyte must be soluble in the solvent.
Generally, the higher the solubility, the higher the capacity of the battery, so it is preferable to have a solubility of about 0.1 to 2.0 M/l. The electrolytes used include lithium perchlorate, lithium hexafluorophosphate, lithium fluorosulfonate, lithium hexafluoroasrate, lithium tetrafluoroborate, lithium trifluoroacetate, and the like. Preferably, the electrolyte salt is a salt of a light metal used in the cathode. This is particularly preferred when the battery is a secondary battery. As with the electrolyte, it is preferable to use a highly pure electrolyte and dry it under reduced pressure below the decomposition temperature to reduce the water content. Next, for the negative electrode, potassium, sodium, lithium, calcium, magnesium, barium, aluminum, zinc, etc. can be used alone or in an alloy. As for its manufacturing method, methods such as pressing the negative electrode metal onto another metal or carbon support or electrolytically depositing it can be used. Batteries can be made into paper-thin cells, stacked with multiple layers connected in series or parallel to each other, or shaped into a circle or spiral. Each characteristic value described in the text is measured by the following method.
This is the calculated value. Pore diameter and pore volume The adsorption isotherm of nitrogen gas at liquid nitrogen temperature was determined for the sample dried at 120°C for 2 hours under reduced pressure, and this was applied using the Cranston-Inkley calculation method (Keio). The relationship between the thickness of the multimolecular adsorption layer and the relative pressure is t (Å) = 4.3 [5/1n (Pa/p)] 1/3. The Frenke-Halsey equation was used. In addition,
The pore volume in the range of 30 to 300 Å in diameter is hereinafter abbreviated as TPV. Electrical resistivity in the longitudinal direction of single fibers Arrange an appropriate number of sampled single fibers,
Both ends are fixed with a conductive adhesive, electricity is applied, and the resistance R (Ω) of the fiber is determined from the voltage and current value between the adhesive. Also, measure the length L (cm) between the conductive adhesives. If the single fiber is bent, determine the actual fiber length using a microscope, etc. Next, remove the fibers, use a microscope to find the total cross-sectional area S (cm ² ) in the direction perpendicular to the fiber direction, and use the following formula to calculate the electric specific resistance p in the fiber direction.
Calculate (Ω・cm). p=R・S/L However, the measurement shall be performed in a dry atmosphere at room temperature and relative humidity of 5% or less after drying as in the previous section. Comparative Examples and Examples Twill fabrics were produced using spun yarns made of regenerated cellulose fibers with a single fiber of 2.0 d. This fabric was impregnated with 10% diammonium phosphate based on the weight of the fiber by dipping the fabric in an aqueous solution of diammonium phosphate, squeezing it and drying it.
The activated carbon fiber fabrics A and B were then heated for 90 minutes from 270°C to 850°C, and then activated in nitrogen gas containing 40 vol% of water vapor for different times. Obtained. The TPV of A and B are respectively
0.08cc/g and 0.15cc/g, and the specific resistance of the single fibers is 1.5 x (10 ^-1 ) Ω・cm and 2.2 x (10 ^-1 ) Ω・cm, respectively.
It was hot. Further, a part of Fabric B was immersed in an aqueous solution of magnesium acetate, dehydrated and dried to impregnate magnesium acetate equivalent to 1.9 wt% of the fibers as magnesium, and heated at 100°C in a nitrogen gas stream containing 40% by volume of water vapor. The temperature was raised to 850°C, held for 35 minutes, cooled in a nitrogen stream, and washed with acid and water to obtain activated carbon fiber fabric C. The TPV of fabric C is
0.42cc/g, and the electrical resistivity of the single fiber is 2.7×
(10 ^)-1 Ω・cm. Fabric C was heat treated at 1100℃ in an argon stream to obtain fabric CH, which had a TPV of 0.42.
The cc/g and C remained the same, but the electrical resistivity was 0.92.
× (10 ^-3 ) Ω・cm, which was extremely low. Four types of fabrics A, B, C, CH obtained in this way
100 mg of each fabric was cut out and used as a positive electrode material, and the battery characteristics when using each fabric were measured. The negative electrode used was a nickel mesh with lithium electrolytically deposited in an organic electrolyte. 0.24mm between positive and negative poles
Sandwich a piece of glass fiber paper with a thickness of
The battery was assembled by immersing it under vacuum in an electrolyte solution in which 0.8 M/l of lithium perchlorate was dissolved in a mixed solvent containing 50 parts by volume each of propylecarbonate and 1,2-dimethoxyethane, and then sealing with argon. All assembly was performed in an argon dry box to avoid contamination of air and moisture. Connect the porous carbon fiber electrode and lithium electrode to the positive and negative electrodes of a DC power source, respectively, and
Charging and discharging was performed at a constant current of 100mA. Charging stopped when the battery voltage reached 4.3V. The temperature of the battery was set at two levels: +21°C and -15°C. Table 1 shows the values of discharge capacity and internal resistance at each temperature as a ratio based on the battery of fabric B. The discharge capacity was determined by calculating the capacity up to 3.3V and expressed as a ratio. The better the voltage stability, the higher the actual discharge capacity. As is clear from the table, the battery according to the present invention is
It can be seen that this is an extremely excellent secondary battery with low temperature dependence of discharge characteristics, low internal resistance, and low temperature dependence. Although specific examples of the present invention have been described, the present invention should not be considered limited to those examples.

【table】

[Table] 〓Note〓
Example (fabric used C, CH)
Comparative example (fabric used A, B)

Claims (1)

[Claims] 1. In a non-aqueous battery comprising a light metal negative electrode, a separator, a carbon material positive electrode, and an organic electrolyte, the carbon material has a pore volume of 0.30 Å with a pore diameter of 30 to 300 Å.
A non-aqueous battery using an aggregate of activated carbon fibers having cc/g or more. 2. The non-aqueous battery according to claim 1, wherein the carbon material has an electrical resistivity of 5×10 ⁻² Ω·cm or less.