JPH10312793A - Lithium ion secondary battery positive electrode and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To miniaturize a battery and increase the battery capacity by sticking titanium nitride to the fiber surface of a carbonaceous fiber aggregate porous body, reinforcing and stabilizing an electrode shape, applying electronic conductivity to the fine parts in the porous body, and fitting and forming titanium disulfide crystals on the electronic conductive fiber surface as a positive electrode active material. SOLUTION: A carbonaceous fiber aggregate porous body sample 3 is fixed in a reaction container 1, and the temperature of a high-temperature electric furnace 2 is increased. The mixed gas of hydrogen and nitrogen is passed through a titanium tetrachloride saturater 7, it is stored in a reservoir tank 6 while its titanium tetrachloride concentration is adjusted, and it is fed into a reaction container 1 as pressure pulses. The reaction temperature and raw material gas are changed in the same reaction container 1, and titanium disulfide is deposited in the second stage deposition process. A molding of inexpensive vegetable fibers is smoked and carbonized to form a carbonaceous fiber aggregate porous body, pulse deposition processes of two stages are applied to its surface with two kinds of reaction gases, the degree of freedom in shape and size is increased, and a large capacity and high performance can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a small-sized, high-capacity, high-performance positive electrode for a lithium ion secondary battery, and a method for producing the same.

[0002]

2. Description of the Related Art Most of the positive electrode active materials currently on the market of lithium ion secondary batteries have replaced cobalt dioxide (CoO ₂ ), lithium cobaltate (Li _x CoO ₂ ) powder or expensive cobalt with some other metal. Powder is used,
These powders can be filled directly into the current collector mesh,
Alternatively, electrical coupling is achieved by mixing with a powder having high electron conductivity and solidifying. As other active materials, the use of titanium disulfide layered crystals capable of accommodating a large amount of lithium ions between layers has been attempted, and the crystal powder is filled in a current collector mesh as described above, or a chemical vapor deposition is performed on the electrode surface. There is known a method of precipitating a crystal by the method.

[0003] As a positive electrode of a high performance lithium ion secondary battery, a lithium ion secondary battery has a small size and a high capacity, and at the same time, lithium ions move quickly from an active material to an electrolyte during charging, and conversely, from an electrolyte to an active material during discharging. Although it is necessary to move quickly, the contact between the electrolyte and the active material is not uniform in the structure where the active material is solidified or filled in the mesh, and the transfer of lithium ions to the active material in deep parts is limited. And reduce the utilization rate of the active material. Also, the method of depositing crystals on the electrode surface by chemical vapor deposition has a disadvantage that it is difficult to increase the battery capacity with a small size, and cannot meet the demand for miniaturization.

[0004]

Accordingly, the present invention is directed to a lithium-to-electrolyte solution in which a porous, current-collecting end portion of a fiber-assembled porous material having a high specific surface area and high electron conductivity is produced in detail, and a positive electrode active material is deposited on the surface. An object of the present invention is to provide a new high-performance and stable lithium-ion secondary battery positive electrode capable of easily entering and exiting ions and having a large battery capacity, and a method for producing the same.

[0005]

For this reason, the present invention provides a carbonaceous fiber-assembled porous body coated with titanium nitride to strengthen and stabilize the shape of an electrode and to provide good electronic conductivity to details in the porous body. A positive electrode of a lithium ion secondary battery, characterized in that titanium disulfide crystals are deposited on a conductive fiber surface as a positive electrode active material, and an organic or carbonaceous fiber assembly having a desired electrode shape is produced as a method for producing the positive electrode. The material is carbonized by smoking or the like to form a carbonaceous fiber aggregated porous body, which is placed in a gas phase reactor, and a raw material gas that produces titanium nitride by a chemical reaction is pulsed several thousand times or more at high temperature in the apparatus. By repeating supply and exhaust, a thin layer of titanium nitride is deposited on the surface of the porous fiber, and then the raw material gas that generates titanium disulfide by a chemical reaction is supplied in the same manner as a pulse of several thousand times or more. Care repeated on the titanium nitride deposition fiber surface, characterized in that to precipitate depositing titanium disulfide layered crystal.

[0006]

In the method of the present invention, first, an organic or carbonaceous fiber aggregate having a desired shape and a large number of fine inter-fiber voids is preliminarily molded by various known methods. Materials include carbon fiber, paper, pulp, recycled paper intermediate mud,
Most fibers that maintain their original shape by pyrolysis and are carbonized, such as cotton cloth, absorbent cotton, and thermoplastic resin fibers, can be used.
These fibers are formed into a desired electrode shape together with a small amount of binder, and this is smoked and carbonized in a non-oxidizing atmosphere to obtain a porosity of 80% or more, which inherits the characteristics of the original shape, and a pore diameter between fibers. A porous carbonaceous fiber aggregate having a low bulk density and a high specific surface area of about 10 to 50 μm.

[0007] The carbonaceous fiber aggregated porous body is placed as a substrate in a gas phase reactor capable of applying a high-temperature vacuum, and titanium nitride is applied to almost all fiber surfaces of the porous substrate by a chemical reaction as a first-stage deposition step. This is performed to stabilize the shape of the negative electrode, which is a fiber-assembled porous body, in the final product battery, and to ensure electrical continuity at the current collecting end within the complicated intricate voids. It is necessary to adhere to the carbon fiber surface inside the void, and the raw material gas must always be supplied as an intermittent pulse. Although various kinds of carbides and nitrides having good electrical conductivity are possible, the results of experiments show that titanium nitride is the most excellent in terms of uniformity of deposition, strength, and conductivity. Titanium tetrachloride (TiC)
l ₄ ), a mixed gas of nitrogen and hydrogen is used, and the composition of the gas can be, for example, 1 to 15% of titanium tetrachloride, 5 to 50% of nitrogen, and 35 to 94% of hydrogen.
A mixed gas of ~ 8%, 10-40% nitrogen and 52-86% hydrogen is practically desirable. Ammonia (NH ₃ ) can also be used as a source gas instead of nitrogen.

[0008] The reaction temperature is selected in consideration of the pore diameter and porosity of the carbonaceous fiber aggregate porous body as a substrate. Generally, when the temperature is too high, the deposition rate increases, and the outer surface of the fiber aggregate porous body is increased. This tends to prevent the gas from being supplied into the fine inter-fiber voids, and at low temperatures, the deposition rate is reduced, requiring a long-time treatment. In the case of titanium nitride deposition, the processing temperature is in the range of 750 to 1100 ° C., it is possible to deposit even inside the fine inter-fiber space, but the temperature and temperature of 780 to 950 ° C. generate titanium nitride in terms of properties and efficiency. Is preferred.

The intermittent supply of the raw material gas is instantaneously performed in a reaction vessel evacuated, and the fine inter-fiber voids of the carbonaceous fiber aggregate porous body serving as the base are instantaneously supplied with fresh raw material gas. Since it is filled to normal pressure, even if a large number of substrates are loaded into the reaction vessel at one time, the precipitation of titanium nitride in all the inter-fiber voids occurs almost uniformly. In practice, the step of instantaneously introducing the source gas into the reaction section, the step of depositing while holding the source gas for a required time, and the step of removing the residual unreacted source gas from the inside of the reactor including the inside of the inter-fiber space of the porous body. It is necessary to repeat an extremely short pulse in which three steps of the evacuation step of the hydrogen chloride gas generated by the reaction are defined as one cycle. 1
The switching of the stroke in the cycle is performed by controlling the opening and closing of the solenoid valve 5 according to a preset program.

The instantaneous introduction of the source gas to normal pressure is 0.1
It is completed within seconds, but the holding time for the reaction precipitation needs to be adjusted depending on the reaction temperature.
A short time of about 3 seconds is sufficient, and when the temperature is low, it is desirable to lengthen it to about 2 seconds. The time required for the next evacuation is 5 Torr from the entire reaction vessel including the inside of the interfiber space.
Since the temperature of the substrate is temporarily lowered by an endothermic reaction when the gas is removed to the following pressure state, when a large number of substrates are loaded in the reaction vessel at once, the entire substrate is used as a reference. It takes time to recover to the temperature. Taking these factors into account, the evacuation time is generally 0.
It is set in the range of 3 to 2 seconds. As a result, the required time per pulse is about 1.4 to 4.0 seconds.

Since a very small amount of such a pulse is deposited at one time, it is necessary to ensure electrical conduction to the current collecting end in the gap and to achieve the purpose of stabilizing the shape of the negative electrode. , Thousands or more pulses are required, and
Approximately 0000 pulses are preferred. At the stage where the carbonaceous substrate is coated with titanium nitride in this manner, the intervals of the titanium nitride stitches are 50 μm or less, often distributed in 10 to 20 μm, and the surface area by the BET method is 40 to 100 m ² / g.
Can be very large.

The second stage deposition of titanium disulfide is also performed by pressure pulses in the same reactor as in the first stage deposition process. The porous fiber aggregate is composed of complicated and complicated fibers and has a very high specific surface area. The highly conductive precipitates such as titanium nitride in the first step are not only formed on the surface of the carbonaceous fiber but also at the connection between the carbonaceous fibers. Therefore, the second stage deposition of titanium disulfide is connected to the collector end at every deposition location. Titanium disulfide is a crystal having a hexagonal layered structure, and Li ions are intercalated between layers to form an interlayer compound Li _x TiS _2.
(0 ≦ x ≦ 1). Since Li ions enter and exit from the end of the layer, the layer surface of the titanium disulfide layer structure is deposited at various angles on the curved surface of the fine fiber, and if the reaction conditions are selected, the proportion of the end of the titanium disulfide layer structure exposed It is ideal for lithium ions to enter and exit between the layers of titanium disulfide, and when the positive electrode is immersed in the electrolyte, the contact between the layer structure end and the electrolyte becomes a good fiber-assembled porous structure .

As the source gas for the second stage, a mixed gas comprising titanium tetrachloride, hydrogen sulfide and an inert gas is used. The inert gas is used as a carrier gas for titanium tetrachloride, and argon or hydrogen can be used. When the reaction temperature is 700 ° C. or lower, nitrogen may be used. The composition of the raw material gas is preferably 1 to 7% of titanium tetrachloride, 2 to 14% of hydrogen sulfide, and 79 to 97% of an inert gas, and the molar ratio of (hydrogen sulfide) / (hydrogen sulfide + titanium tetrachloride) is suitable. When the ratio is in the range of 0.4 to 0.9, the deposition rate is large, and the maximum is near the molar ratio of 0.7.
It is desirable to select the molar ratio of The desired precipitation of titanium disulfide occurs at 300 ° C. or higher. At 500 ° C. or lower, titanium trisulfide (TiS ₃ ) and at 900 ° C. or higher, trititanium tetrasulfide (Ti ₃ S ₄ ) and titanium disulfide Since mixed deposition is performed, 600-90
A reaction temperature of 0 ° C. is desirable.

In the second stage deposition step, in order to increase the capacity, it is necessary to increase the number of pulses to increase the amount of titanium disulfide, which is the positive electrode active material, to be deposited inside the interfiber space.
Normally, 5,000 pulses or more are required. For this purpose, it is necessary that the vent of the reaction gas into the inter-fiber space be opened until the last time of the pulse, and the reaction temperature is lowered to prevent deposition on the outer surface of the substrate. Must be suppressed. The porous fibrous body is composed of complicated and intricate fibers, has a high specific surface area, and is closely connected with a highly conductive titanium nitride film. Even if titanium sulfide, that is, the positive electrode active material precipitates to 60% or more of the inter-fiber void volume, the contact between the positive electrode active material and the electrolytic solution can be maintained, and a sufficient space for lithium ions to enter and leave can be left.

The shape of the entire positive electrode of the secondary battery is basically a flat plate, but other shapes such as a corrugated plate, a cylinder, and a spiral are naturally possible. In particular, since the electron conductivity is high up to the fiber end of the electrode, several units can be combined and integrated. Since the assembly can be performed even after the deposition and deposition of the positive electrode active material titanium disulfide, an extremely complicated combination in units of simple electrodes is possible.

Hereinafter, the production method of the present invention will be described with reference to a laboratory example, but the present invention is not limited thereto.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an outline of an apparatus for executing the method of the present invention. This apparatus is a small-scale experimental apparatus, and a scale-up of several hundred times can be used for actual production. Also,
In this embodiment, a method of inserting the quartz glass reaction vessel 1 into the furnace for controlling the furnace atmosphere of the high-temperature furnace 2 is adopted. However, it is of course possible to directly use a large vacuum atmosphere furnace.

First, a commercially available filter paper was lightly adhered to three layers, and 20%
What was immersed in an alcohol solution of phenyl resin and dried was carbonized at 1000 ° C. for 3 hours in an argon stream to obtain a thin plate-like carbonaceous fiber aggregate having a thickness of about 1.2 mm. Figure 2 shows the average microstructure by SEM observation.
(A). The carbonized fibers having a thickness of about 10 μm were intertwined to form a fragile thin plate-like fiber aggregate having an overall porosity of 85%.

This was further cut into 10 × 20 mm, fixed as a carbonaceous fiber aggregated porous sample 3 in the reaction vessel 1, the electric furnace 2 was heated to 870 ° C., and a mixed gas of hydrogen and nitrogen was tetrachloridized. After passing through a titanium saturator 7, the concentration of titanium tetrachloride was adjusted to 5%, stored in the reservoir tank 6, and then supplied as a pressure pulse into the reactor, and this was performed up to 15,000 pulses. As a result, the electrical resistivity of the fibrous carbon substrate was from 0.19 Ωcm before titanium nitride deposition to 200
0.019 Ωcm after 0 pulse, 0.0015 Ωcm after 10000 pulse, 0.000 after 15000 pulse
It became 4 Ωcm, and it was shown that the resistance value had already sharply decreased at 2000 pulses, and further decreased gradually thereafter. FIG. 2 (b) shows a sample of FIG.
The microstructure after deposition up to 000 pulses is shown.

Next, in the same reaction vessel, the reaction temperature and the source gas were changed, and the titanium disulfide was vapor-deposited in the second vapor deposition step as follows. The reaction temperature was set to 1000 ° C., and as a raw material gas, a 1: 1 mixed gas of hydrogen and argon was passed through a titanium tetrachloride saturator 7 and stored in another hydrogen sulfide gas and a reservoir tank 6. The composition of this source gas is titanium tetrachloride 4
%, 10% hydrogen sulfide. From this, a raw material gas was introduced as a pulse into the reactor 1 at 800 ° C. to precipitate titanium disulfide hexagonal layered crystals in the titanium nitride-coated fiber aggregate. When the holding time per pulse is 0.8 seconds, the evacuation time is 1 second, and 25,000 pulses are performed, the volume of titanium disulfide is filled up to about 63% of the void volume after the first-stage deposition. There was no change with increasing.

When the same pulse deposition is performed with the reaction temperature lowered to 600 ° C., the deposition in the voids in the fiber-assembled porous material is reduced to 40 ° C.
Up to 000 pulses, up to 72% of the void volume could be filled with the volume of titanium disulfide. FIG. 2 (c) shows the microstructure of the sample of FIG. 2 (b) when vapor deposition was performed up to 20,000 pulses. The average diameter of the titanium disulfide layered crystal deposited on the surface is about 130 μm at 600 ° C. and 800 μm.
In the case of ° C., it was about 60 μm. Also, the electric conductivity was somewhat increased by the deposition of titanium disulfide, and the resistivity after the deposition was reduced by about 10% from that before the deposition.

[0022]

According to the present invention, an inexpensive molded article such as vegetable fiber is smoked and carbonized to form a carbonaceous fiber aggregated porous body, which is used as a base material, and a two-stage pulse is formed on the surface thereof with two types of reactive gases. By performing the deposition process, a large amount of hexagonal layered crystal of titanium disulfide is deposited on the surface of the metal conductive titanium nitride at the current collecting end, and lithium ions can easily enter and exit the layered structure. A high capacity, high performance, high performance secondary battery positive electrode is obtained. In the method of the present invention, vapor deposition is possible even if a large number of carbon substrates are filled in a reactive vapor deposition container, so that there is no difficulty expected when the production apparatus is enlarged, and mass production is possible industrially. It is of extremely high practical value.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing the arrangement of an example of an intermittent chemical vapor infiltration reactor used for carrying out the present invention.

FIGS. 2A and 2B are explanatory diagrams of a change in microstructure showing a process of forming a positive electrode of a lithium ion secondary battery of the present invention. FIG. 2A shows a carbon fiber aggregate porous body serving as a substrate, and FIG. (C) is a view showing a state in which titanium disulfide is further deposited thereon.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Reaction container 2 High temperature electric furnace 3 Carbonaceous fiber aggregate porous sample 4 Vacuum pump 5 Solenoid valve 6 Reservoir tank 7 Titanium tetrachloride saturator

Claims (2)

[Claims] 1. Titanium nitride is deposited on the surface of a fiber of a porous carbonaceous fiber assembly to strengthen and stabilize the shape of an electrode and to provide electronic conductivity to the details in the porous body. A positive electrode for a lithium ion secondary battery, wherein a titanium disulfide crystal is deposited as a positive electrode active material. 2. An organic or carbonaceous fiber aggregate having a desired electrode shape is carbonized by smoking or the like to form a carbonaceous fiber aggregate porous body, which is placed in a gas phase reactor, and titanium nitride is chemically reacted. A thin layer of titanium nitride is deposited on the surface of the porous fiber by repeatedly supplying and exhausting the raw material gas as a pulse of several thousand times into the apparatus at a high temperature, and then producing titanium disulfide by a chemical reaction A method for producing a positive electrode for a lithium ion secondary battery, comprising repeatedly depositing and depositing titanium disulfide layer-like crystals on the surface of a titanium nitride-coated fiber by repeating supply and exhaust as thousands of pulses or more using the same source gas. .