JPH04342958A - Negative electrode for battery with non-aqueous electrolyte - Google Patents

Abstract

PURPOSE:To construct a negative electrode equipped with a high energy density and high-rate discharging capability, by forming negative electrode from spherical carbon particles of lamellar structure in which carbon molecle chains are overlapped in layers in the C-axis direction. CONSTITUTION:A positive electrode is installed inside a pos. electrode vessel 1 made of stainless steel, and upon this positive electride 2, a separator 3 of non-woven clot and fine porous film, negative electrode 4, and pressing plate 5, are placed one over another. At the opening of this vessel 1, a neg. electrode vessel 7 made of stainless steel is installed through a packing 6, and thereby the positive electrode 2, separator 3, negative electrode 4, and pressing plate 5 are put enclosedly in the vessels 1, 7. In the neg. electrode vessel 7, an electrolyte, which is formed by dissolving LiClO4 in a 3:1 mixed solution of acetonitrile and ethylene carbonate, is contained. The negative electrode 4 is formed from carbon particles of lamellar structure in which carbon molecle chains are overlapped in layers in the C-axis direction, and the paste containing 24 parts by wt. carbon 24 and one part by wt. polytetrafluoroethylene is dried, molded in a net made of stainless steel, enwrapped in the condition as it is, and embodies in a disc.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] The present invention relates to a negative electrode for non-aqueous electrolyte batteries, and more particularly to a new negative electrode for non-aqueous electrolyte batteries that is capable of high capacity, high energy density, high rate discharge, and has excellent charge/discharge cycle characteristics. .

0002

BACKGROUND OF THE INVENTION Various materials have been studied as negative electrode active materials for non-aqueous electrolyte batteries, but lithium-based negative electrodes are attracting attention as they are expected to have high energy density. In particular, lithium metal, lithium alloys, carbon holding lithium ions, and the like are being considered as negative electrodes for non-aqueous electrolyte secondary batteries.

Lithium metal has a high electromotive force and can be expected to have a high energy density, but its high reactivity poses problems in battery safety, and dendrites are likely to occur during charging reactions, causing internal short circuits and charging problems. This poses a major problem, such as a decrease in discharge efficiency.

[0004] Lithium alloys can prevent the formation of dendrites, but due to their characteristics, the potential of the alloy shifts in a noble direction with respect to the lithium potential, resulting in a voltage drop. Additionally, because it contains metallic lithium, there were still safety issues.

[0005] In order to improve safety issues, carbon negative electrodes are being considered as host materials that retain lithium ions. A carbon negative electrode holds lithium ions between layers of a crystal lattice, and easily releases lithium ions through a discharge reaction. Carbon negative electrodes are highly safe because they do not use metallic lithium, and they are less susceptible to deterioration due to charging and discharging, making it possible to create long-life non-aqueous electrolyte batteries. Regarding the carbon used at this time, there are various reports such as PAN-based, pitch-based, or rayon-based carbon fibers, vapor-grown fibers, carbon bodies produced by thermal decomposition of specific polymer compounds, carbon bodies produced by pyrolysis CVD method, and activated carbon. has been done. None of these studies has defined the structure of carbon with high capacity and high energy density.

[0006]

[Problems to be Solved by the Invention] Conventional carbon negative electrodes have the drawbacks of low discharge capacity and poor voltage flatness. The discharge capacity is determined by the amount of chargeable and dischargeable lithium ions present between the carbon crystal layers. The distance between crystal layers is a factor that greatly affects the discharge performance of the electrode plate, and this distance can be determined by X-ray diffraction.

In conventional carbon, when the distance between crystal layers becomes short, it becomes difficult for lithium ions to enter between the layers, and the discharge capacity decreases. As the interlayer distance increases, the crystallinity of carbon decreases, and the ratio of the amorphous layer to the crystal layer increases. Since the amorphous layer does not occlude lithium ions, the discharge capacity of the electrode plate becomes small. Therefore, it has become clear that carbon, which maintains appropriate crystallinity and has a long interlayer distance, is excellent as an electrode, and various studies have been conducted.

For example, graphite that exhibits high orientation has a developed layered crystal structure, with short interlayer distances, and
Due to the longer approach distance, it became physically difficult for ions to reach the ion storage site, and the amount of charge and discharge was not large. In PAN-based carbon fibers, when the degree of crystallinity is increased, the amount of discharge decreases for the same reason as graphite, and when the degree of crystallinity is decreased, the functional groups on the carbon surface inhibit charging and discharging, making it impossible to discharge. Pitch-based and vapor-grown carbon can have a relatively high charge/discharge capacity, but most of them are fibrous in shape, and ions enter from both ends of the fiber, making it difficult for ions to reach deep into the fiber. difficult,
The ion storage sites in carbon could not be fully utilized. Electrodes in which carbon is deposited on the metal surface through thermal decomposition of hydrocarbons are difficult to make thick, and are not suitable for high-capacity electrodes.

[0009] Voltage flatness is better as the degree of crystallinity is higher, but carbon with excellent voltage flatness does not have a large discharge capacity as described above. The present invention provides carbon having a large discharge capacity and excellent voltage flatness.

0010

[Means for Solving the Problems] The carbon negative electrode according to the present invention is characterized in that it is a spherical carbon having a lamellar structure in which molecular chains are layered in the C-axis direction.

The carbon negative electrode according to the present invention is manufactured as follows. The pitches are heat treated at a first treatment temperature. The giant aromatic polymer produced by the polymerization reaction that occurs at that time folds to form thin plate-shaped single crystals, or lamellae, which are stacked to form a structure. This structure is called a lamellar structure. This structure forms spherical particles that precipitate within the pitch and has optical anisotropy. When this spherical polymer compound with a lamellar structure is extracted by treatment with an organic solvent and thermally decomposed at a second treatment temperature in an inert gas such as argon, a spherical carbon body is produced while maintaining the lamellar structure. Ru. This carbon body is the carbon used in the present invention.

[0012] The above first treatment temperature is from 200°C to 60°C.
A temperature of 0°C, preferably 350°C to 400°C is used. The second treatment temperature is 700°C to 3000°C, preferably 1500°C to 2800°C. A structural diagram of carbon having this lamellar structure is shown in FIG. The particle size of this carbon can be freely changed within the range of 1 to 100 angstroms depending on the first treatment temperature. In addition, (002) was determined by X-ray diffraction, which is one of the parameters indicating the crystallinity of carbon.
The average value of the interlayer distance on the surface is within the range of 3.35 angstroms to 3.75 angstroms, and it is possible to change this value.

The ability to control the particle size and crystallinity in this way allows carbon to be adapted to the characteristics of various doped and undoped substances, making it an extremely versatile carbon.

The most important features of this carbon are its spherical shape and lamellar structure.

[0015] The larger the contact area between the inlet between the carbon layers and the electrolytic solution, the easier charging and discharging will proceed, and high rate discharge will be possible. Compared to cylindrical, prismatic, and scale-shaped carbon, spherical carbon has a wider space for the electrolyte to enter when it is made into a plate, so the contact area mentioned above is wider, and it is better at high rate discharge than other carbons. , the discharge capacity also increases.

In general carbon, if the degree of crystallinity is increased, the interlayer distance becomes shorter and the crystallites become longer, making it difficult for ions to enter between the layers.If the degree of crystallinity is decreased, the crystallites become shorter, but the amorphous portion It has already been mentioned that the discharge capacity decreases due to the increase in . The carbon having a lamellar structure according to the present invention differs from conventional carbon in that it has a broad X-ray diffraction peak. This is thought to be due to the fact that the crystals constituting the carbon having a lamellar structure overlap in layers like fallen leaves, and there are parts where the distance between the layers is wide. This can be estimated from the fact that, as shown in the X-ray diffraction diagram of FIG. 2, those with a lamellar structure have a gentle peak on the (002) plane.

As charging progresses, ions begin to intercalate from areas where the interlayer distance is wide, and ions move to areas where the interlayer distance is short. As the ions move, the part with the short interlayer distance is widened, but the widened distance is absorbed by the part with the long interlayer distance, so that the ions can be held stably as a whole. Similarly, ions can be stably emitted during discharge. Therefore, despite the high crystallinity, the discharge capacity is large, and the discharge curve has voltage flatness close to that of graphite. By using this carbon negative electrode, it has a high energy density and is compact.
We were able to provide a lightweight, high-rate discharge battery.

[0018]

[Function] According to the present invention, the carbon used for the negative electrode is preferably
surface) is 3.35 angstroms or more and 3.75 angstroms or less, and the appearance is preferably 1 μm in diameter.
It has a true spherical shape with a diameter of 100 μm, and the inside thereof has a lamellar structure in which carbon molecular chains are stacked in layers in the C-axis direction.

[0019] Compared to conventional carbon or graphite, carbon with this lamellar structure can intercalate and deintercalate ions from any direction on the plane perpendicular to the C axis, and has a particle-like shape. Therefore, it can be said that the electrolyte retention property is good. In other words, the contact surface between carbon and electrolyte has many entrances where ions intercalate and deintercalate, and since there is a large amount of electrolyte containing ions around these entrances, ions can be easily intercalated. Since carbon can be deintercalated and ion storage sites between carbon crystal layers can be effectively utilized, the charge/discharge capacity per unit weight of carbon can be greatly increased.

[0020]

EXAMPLES An example in which the present invention is applied to a button type non-aqueous solvent secondary battery will be described in detail below with reference to FIG. [Embodiments of the Present Invention] In this embodiment, carbon particles having particle diameters of 20 μm and 40 μm and an interlayer distance of 3.36 angstroms were used.

Reference numeral 1 in the figure indicates a stainless steel cathode container, and a cathode 2 is disposed inside the container. Positive electrode 2 is manufactured as follows. First, it was fired at 900℃.
100 parts by weight of LiCoO2 powder, 5 parts by weight of liquid polytetrafluoroethylene, and 3 parts by weight of carbon black are made into a paste using ethyl alcohol. It is then dried, molded in a stainless steel net, and wrapped as it is. In this way, the density of the electrode plate was reduced to 2.6g/
A cm3 disc-shaped electrode was prepared and used as a positive electrode. On top of this positive electrode 2 is a separator 3 made of nonwoven fabric and a microporous membrane.
, a negative electrode 4 , and a compression plate 5 are arranged in this order. A stainless steel negative electrode container 7 is provided at the opening of the positive electrode container 1 through a packing 6, thereby sealing the positive electrode 2, the separator 3, the negative electrode 4, and the compression plate 5 in the positive electrode container 1 and the negative electrode container 7. There is. Positive electrode container 1
In the negative electrode container 7, 1 m of LiClO4 was added to a 3:1 mixed solution of acetonitrile and ethylene carbonate.
An electrolytic solution dissolved at a concentration of ol/l is stored. Negative electrode 4 is manufactured as follows. 24 parts by weight of the spherical carbon of the present invention and 1 part by weight of liquid polytetrafluoroethylene are made into a paste using ethyl alcohol. It is then dried, molded in a stainless steel net, and wrapped in the same way as the positive electrode. In this way the density 1.1
A disc-shaped electrode of g/cm3 was prepared and used as a negative electrode.

[0022] The button type non-aqueous solvent secondary battery thus produced was subjected to a charge/discharge test. The amount of active material charged is about three times the electrochemical equivalent of the positive electrode than the negative electrode.
The battery capacity is limited to the negative electrode. Further, the current density for charging and discharging the battery was set to 0.5 mA/cm2. The results of the cycle test are shown in FIG. 4, and the discharge curve is shown in FIG. 5. In the cycle test, the end-of-charge voltage was 4.0V, and the end-of-discharge voltage was 2.
．． It was set to 8V. As a result, the battery according to this example exhibited a discharge capacity of 250 mAh/g per weight of carbon. [Comparative Example] As Comparative Example 1, the crystal interlayer distance (002 plane)
is 3.49 angstroms, and the density of the electrode plate is 1.2
g/cm3 vapor grown carbon fiber, as Comparative Example 2,
Comparative Example 3 is a pitch-based carbon fiber with a crystal interlayer distance (002 plane) of 3.67 angstroms and an electrode plate density of 1.5 g/cm3. Density is 1.9g/
cm 3 of graphite was used for the carbon negative electrode, and batteries were produced in the same manner as in the example except for the following.

[0023] The obtained battery was subjected to a cycle test under the same conditions as in the example. Comparative Example 1 using vapor-grown carbon fiber showed an initial discharge capacity of 200 mAh/g, Comparative Example 2 using pitch-based carbon fiber 150 mAh/g, and Comparative Example 3 using graphite 50 mAh/g. . In graphite, the capacity decreased as the cycles progressed, and the capacity disappeared after about 30 cycles. From the discharge curve,
Voltage flatness was observed in graphite, but this was not observed in vapor-grown carbon fiber and pitch-based carbon fiber.

[0024]

[Effects of the Invention] The negative electrode for non-aqueous electrolyte batteries of the present invention has a high energy density, is capable of high rate discharge, and has excellent voltage flatness, and is extremely useful.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the internal structure of carbon particles.

FIG. 2 is a diagram showing an X-ray diffraction diagram of carbon.

FIG. 3 is a cross-sectional view of a button-type non-aqueous solvent secondary battery showing one embodiment of the present invention.

FIG. 4 is a characteristic diagram showing the relationship between cycle number and discharge capacity in Examples and Comparative Examples.

FIG. 5 shows discharge curves of the first cycle of Examples and Comparative Examples.

[Explanation of symbols]

1. Positive electrode container 2　Positive electrode plate 3 Separator 4 Negative electrode plate 5. Compression plate 6 Packing 7 Negative electrode container

Claims (1)

[Claims] 1. A negative electrode for a non-aqueous electrolyte battery, characterized in that it uses spherical carbon particles having a lamellar structure in which carbon molecular chains are layered in the C-axis direction.