JPH0461747A - Negative electrode for lithium secondary battery - Google Patents

Abstract

PURPOSE:To enhance the stability of the discharge capacity for repeated charging/discharging by using carbon fiber made from mesophase pitch as starting material, and therefrom selecting specific carbon fiber. CONSTITUTION:A neg. electrode concerned uses an active substance which is carbon fiber of specific structure. That is, the carbon fiber has properties such that the lattice surface spacing (d002) as crystallite parameter due to X-ray diffraction method ranged 0.338-0.343nm, the size (Lc) of the crystallite in the c-axis direction ranges 10-30nm, the orientation factor (FWHM) ranges 7-20deg., and the mean length in fiber cross-section direction of the graphite crystallite stretching long in the fiber axis direction ranges 20-100nm. In this carbon fiber, the graphite crystallite is surrounded by amorphous portion, which absorbs expansion/contraction of the graphite crystallite elastically, so that macro- structural destruction of carbon fiber will not generated. This provides a large discharge capacity and ensures high stability of discharge capacity for repeated charging/discharging.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to negative electrode materials for non-aqueous electrolyte lithium secondary batteries. More specifically, the present invention relates to a negative electrode material suitable for high-performance non-aqueous electrolyte lithium secondary batteries that require long life and high energy density.

Conventional technology: Batteries using alkali metals as negative electrode active materials have many advantages, such as high energy density, light weight, small size, and excellent long-term storage stability due to the use of non-aqueous electrolytes. It has been put into practical use as a lithium secondary battery and is widely used.

However, when this negative electrode active material is used as a negative electrode of a secondary battery, new problems arise that do not exist in secondary batteries. That is, secondary batteries using lithium as a negative electrode have short charge/discharge cycle life and low discharge efficiency. This is because lithium deposited on the negative electrode during charging forms so-called tentolites (dendrites).

As a solution to these problems in turning lithium into secondary batteries, the following methods are being considered. That is, improving the electrolyte, using an alloy containing lithium for the negative electrode, using a conductive polymer for the negative electrode, and using a carbon material for the negative electrode.

To improve the mM solution, the combination of organic solvents and the addition of small amounts of solvents (e.g., VT vapor chemistry and industrial physical chemistry, 57, 52
3 (1989)) is being considered. These methods are
This method relies on the fact that the state of lithium precipitation during charging changes depending on the organic solvent, but at present no electrolytic solution has been found that can completely eliminate the formation of dendrites.

As alloys containing lithium, for example, Li-M alloys, wood alloys, etc. are being studied (National Te
ch, Report, Vol, 32. No,
5 (19813)), and by using these alloys in the negative electrode, the precipitated lithium is alloyed and incorporated into the electrode, so that in principle, the growth of dendrites can be eliminated. However, expansion and contraction of the electrode occurs due to changes in the crystal structure of the alloy depending on the concentration of lithium, so charging and discharging with a large change in C degree of lithium is not possible, and diffusion of lithium into the interior of the alloy is the rate determining factor. Therefore, there are problems such as the inability to increase the current density.

In addition, conductive polymers apply lithium doping and dedoping to negative electrode reactions, and since lithium is doped into the polymer without being precipitated, the generation of dendrites can be theoretically eliminated (solid-state physics, 17.793 (198
2)). However, the potential of lithium-doped polymers is 1.2 V (lithium standard), which is significantly lower than lithium, and it is not possible to increase the number of lithium-doped products because of the deterioration of the charge/discharge cycle life. Other issues remain.

As mentioned above, the current situation is that the three types of means have not necessarily succeeded in solving the drawbacks of the negative electrode. A new method that has recently attracted attention as an alternative to these methods is the use of carbon materials as negative electrode active materials. By using a carbon material for the negative electrode, lithium is incorporated between the carbon layers during charging (intercalation reaction), forming a so-called graphite intercalation compound. Therefore, in principle, the generation of dendrites can be eliminated.

In addition, negative electrodes made of carbon materials are chemically more stable with respect to lithium metal than those made of lithium-containing alloys or conductive polymers, and are theoretically capable of incorporating lithium up to -Li. is possible, and the lightweight nature of carbon materials
It has advantages such as being able to increase the energy density of the negative electrode material in combination with good conductivity, and also because of the easy reversibility of the essential reaction, which is a characteristic of intercalation reactions.
It has a long life even after repeated charging and discharging, and is expected to be used as a negative electrode for secondary batteries.

As a carbon material suitable for the negative electrode of a lithium secondary battery having the above-mentioned characteristics, the inventors of the present application have found that carbon fiber made from a pitch defined by X-ray diffraction and Raman spectroscopy is excellent. However, when charging and discharging are repeated using this pitch-based carbon fiber as a negative electrode, the discharge capacity tends to decrease with cycles. was sometimes observed, and it was necessary to improve the cycling stability of the capacity.

Problems to be Solved by the Invention The present invention has been made in view of the current situation as described above, and provides a pitch suitable for negative electrodes for nonaqueous electrolyte-based lithium secondary batteries, which has a highly stable discharge capacity against repeated charging and discharging. The purpose is to provide carbon fibers based on carbon fibers.

Means to Solve the Problems In order to solve the above-mentioned problems, the authors of the IJI have conducted extensive studies on the structure of pitch-based carbon fibers, and as a result, they have created graphite crystallites of an appropriate size and their surroundings. We have discovered that the composite structure, which is an aggregate of amorphous parts, is essentially important for the cycle stability of lithium secondary batteries. Furthermore, for the purpose of specifying this composite structure, in addition to the X-ray diffraction index used to specify carbon materials for batteries, orientation coefficients that specify the size of crystallites and carbon It has been discovered that two indicators, the size of graphite crystallites in the cross-sectional direction of m-fibers, are optimal for specifying carbon fibers, leading to the present invention.

That is, the present invention relates to a lithium secondary battery using an organic electrolyte in which a lithium salt is dissolved in an organic solvent, and a negative electrode characterized in that carbon fiber defined below is used as an active material. That is, the lattice spacing (d002), which is a crystallite parameter determined by X-ray diffraction, is 0.338 nm.
0.343 or more, the crystallite size in the C-axis direction (Lc) is IQam or more and 3 (lnm or less), and the orientation coefficient (
FWHM) is 7° or more and 20° or less, and the average length of graphite crystallites elongated in the fiber axis direction in the fiber cross-sectional direction is 20n layers or more and 100 nm or less.

The method of expressing the various physical property values mentioned above used in the specification of carbon fibers is explained below: (1) X-ray diffraction method Using CuKα as the X-ray source, high-purity silicon as the standard material, and The 002 diffraction pattern was measured, and the lattice spacing d was determined from the peak position and half-value width. 02. The average length Lc of graphite crystallites in the C-axis direction is calculated. The calculation method is, for example, "Carbon Fiber" (Kaidai Editsha, 1988)
Published in March 2016), pages 733-742.

(2) Orientation coefficient based on X-ray diffraction of carbon fibers A detector that irradiates X-rays perpendicularly to a plane containing straight carbon fibers and sets the transmitted diffracted light in the direction where the 002 plane diffraction is maximized. Detect with. With the X-ray diffraction measurement system fixed under these conditions, when the mwr bundle is rotated in a plane perpendicular to the incident X & 9 and the diffracted light is measured, the diffraction pattern becomes a periodic function of the fiber rotation angle of 180'', and - One peak per period is 71<, and the orientation coefficient is the half width (FWH) of this peak.
M) is defined as

(3) Measurement is made using a transmission electron mirror (TEM) as described below.

After embedding the carbon fibers in the resin, thin pieces are obtained using, for example, a diamond cutter. At this time, the surface of the thin piece and the carbon fiber axis are made parallel. When observing a 002 dark-field image of a thin section using a transmission electron microscope, a lattice pattern of white bands and black bands parallel to the fiber axis is observed. The average length in the cross-sectional direction of the membrane region extending in the mmI axis direction from the average spacing Δ of the lattice fringes and the image magnification A is determined by the following formula: Also = Δ/A Below 1 The negative electrode using the pitch-based carbon fiber of the present invention will be explained in detail.

The raw material peach for spinning used in the present invention has molecular species that are easily oriented and is not limited in any way as long as it provides an optically anisotropic pitch, that is, a membrane pitch. Menface pitch can be used. Carbonaceous raw materials for obtaining these membrane pitches include, for example, coal-based coal tar pitch, petroleum-based heavy oil, pitch, etc. The pitch for spinning in the present invention is basically a material that is easily graphitized. It is desirable that the membrane content is 70% or more, preferably 80%.
As described above, optimally, a pitch containing 85% or more of membrane is suitable.

In order to obtain a pitch-based carbon fiber suitable for a lithium secondary battery negative electrode that has high cycle stability against repeated charging and discharging, the present inventors conducted a detailed study on the manufacturing process and evaluation method of carbon fiber. As a result, the essential point in the structure of carbon fiber is to suppress the development of graphite crystal structure by subdividing the easily graphitizable membrane pitch during manufacturing processes such as spinning. It has been found. Fibers with an internal structure of subdivided mesophases are composed of regions with a developed graphite crystal structure (hereinafter referred to as graphite crystallites) and amorphous parts that interconnect these graphite crystallites. It is. That is, it has been found that a composite structure consisting of graphite crystallites and an amorphous portion that connects them is the optimal structure for a negative electrode for a lithium secondary battery.

The reason why the carbon fiber having a composite structure of crystallites and an amorphous part in the negative electrode for lithium secondary batteries as claimed by the present invention has excellent characteristics as a negative electrode active material for lithium secondary batteries will be explained below. .

During charging, lithium is inserted from the electrolyte into the width of the carbon fiber that serves as the negative electrode. Lithium within the carbon fiber is selectively inserted into graphite crystallites that have developed energetically stable crystal structures. By inserting lithium, the interplanar spacing of the carbon layer planes that make up the graphite crystallite expands, and at the same time, the graphite crystallite expands, but this expansion is elastically absorbed by the amorphous part surrounding the crystallite. , it does not lead to destruction of the macroscopic structure of carbon fiber.

During discharge, the lithium in the carbon fiber is released into the electrolyte. When lithium is released from between the layers, contraction occurs between the layers, but since the amorphous part surrounding the graphite crystallites elastically returns the crystallites to their original state, carbon fiber does not cause any crystal breakage. It is not accompanied by

In other words, the insertion reaction of lithium into graphite crystallites causes expansion of the graphite crystallites, which is normally an irreversible destruction of the crystal structure, but the carbon fiber claimed by the present invention It is surrounded by an amorphous portion, and this amorphous portion elastically absorbs the expansion and contraction of graphite crystallites, thereby achieving reversibility. In addition, in the release reaction of lithium from graphite crystallites, the expanded graphite crystallites contract back to their original state, but this contraction occurs due to the elasticity of the amorphous portion, and the macroscopic carbon fiber structure is not destroyed. Not enough.

That is, the following two conditions are suitable for carbon fibers to be repeatedly charged and discharged stably. One is that there is a part with a developed graphite structure that is a reaction site, and the other is that there is an amorphous part that elastically absorbs expansion and contraction.

- As mentioned above, an important point in the present invention is to suppress the development of graphite crystallites, that is, by dividing the mesoface pitch finely, the crystallites and the non-containing particles surrounding them are forced into the carbon fiber. The aim is to introduce a complex structure called a crystalline part.

When carbon fibers with such a structure are expressed in terms of physical properties, they are as follows: (1) In terms of crystallite parameters determined by X-ray diffraction, the lattice spacing (doo2) is 0.338 nm or more and 0.3
43 n1 or less and the crystallite size in the C-axis direction (
Lc) is 10 nm or more and 30 nm or less.

(2) Orientation coefficient (FWHM) in X-ray diffraction method is 7
More than ° and less than 20'.

(3) The average length in the cross-sectional direction of the graphite crystallites extending in the fiber axis direction is 20 ngg or more and 100 ni <
below.

Among the above conditions, (1) defines the range suitable for a negative electrode in terms of the average degree of graphitization of carbon fibers, and (2) corresponds to the length of graphite crystallites extending in the fiber axis direction. . That is, the longer the length of the graphite crystallites, the stronger the tendency for the graphite crystallites to be aligned parallel to the m#I axis, so that the orientation of the crystallites develops and the orientation angle becomes smaller. (3) defines the size of the graphite crystallite in the cross-sectional direction. (2) , (
3) is a regulation regarding the size of graphite crystallites. In this way, focusing on the internal structure of carbon fiber, Li
We discovered that the essence of the optimal structure for secondary battery negative electrodes is a composite structure consisting of graphite crystallites and amorphous parts.
d by diffraction. o2. The greatest feature of the present invention is that the composite structure can be defined by Lc, the orientation angle of the graphite crystallites, and the size of the graphite crystallites in the fiber cross-sectional direction.

By using carbon fibers defined in this way, it is possible to provide a negative electrode that is stable against repeated charging and discharging.

The carbon fibers defined in the present invention are preferably produced by the following methods alone or in combination.

(1) The thinner the ultrafine fiber raw material pitch is spun, the more it is possible to generally suppress the growth of crystallites in the cross-sectional direction of the carbon fiber, and at the same time, to disturb the orientation of the crystallites. Both effects make it possible to suppress the uniform degree of graphitization of carbon fibers as determined by X-ray diffraction.

The physical properties of specific carbon fibers are not determined only by the diameter of the carbon fibers, but also vary depending on the properties of the raw material pitch, the viscosity of the pitch during spinning, the shape of the spinning nozzle, etc. The diameter of the fibers is 8 m or less.

(2) In the spinning process where mesh filters are used to finely divide membranes, a net (mesh filter) is installed in advance in the flow path of the bead until the raw material pitch is discharged from the spinning nozzle. so that it is subdivided accordingly. The carbon fiber spun in this way is
Within the cross section, the membrane is subdivided according to the mesh filter, so that the development of graphite crystallites is suppressed, and the orientation of the crystallites can also be suppressed. Both effects can suppress the average degree of graphitization of carbon fibers determined by X-ray diffraction.

(3) Fragmentation of Memface by Stirring In the spinning process, a stirrer is installed in advance in the pitch flow path until the raw material pitch is discharged from the spinning nozzle, and the beech is divided into smaller pieces by the rotation of the stirrer. Do it like this. The carbon fibers spun in this manner have finely divided membranes in the cross-sectional direction and in the fiber axis direction, so that the development of graphite crystallites is suppressed, and the orientation of the crystallites is also suppressed. and,
Both effects can suppress the average degree of graphitization of carbon fibers as determined by X-ray diffraction.

The graphitization treatment of carbon fibers is carried out under an inert atmosphere at a temperature of 2000°C or higher, preferably 2600°C.
The above is appropriate. Since the stability of carbon fibers against charge/discharge cycles basically requires a well-developed carbon layer surface, graphitization treatment at a somewhat high temperature is desirable.

The present invention relates to a negative electrode for a lithium secondary battery characterized by using carbon fiber having a structure suitable for the reaction of inserting and releasing lithium as described above, and an electrode using the carbon fiber of the present invention. If there is, there is no restriction on the shape of the electrode.

As the electrolyte for the lithium secondary battery in the present invention, lithium perchlorate (L i Ci 04 ), lithium borofluoride (LiBF+), lithium hexafluoroantimonate (LiSbFG), lithium antimony hexachloride (LiSbCijb), 67-/Lithium hexafluoride (LiAsF6), Lithium hexafluorophosphate (LiP
Lithium salts such as Fr, ), lithium trifluoromethylsulfonate (LiCF3SO3) are preferably used. What is basically required of the lithium salt used as an electrolyte is the stability of the anion in electrochemical reactions, so as long as this condition is satisfied, the lithium salt used as an electrolyte is limited to the above-mentioned lithium salt. isn't it.

The negative electrode using carbon fiber provided by the present invention is put into practical use by being incorporated into a lithium secondary battery as a negative electrode by a conventional method. That is, as the positive electrode, for example, a chalcogen compound of a transition metal, a conjugated polymer compound, or activated carbon can be used. A nonwoven or woven fabric made of synthetic fibers, a nonwoven fabric or woven fabric made of glass fiber, or the like can be used as a separator between the negative electrode and the positive electrode. These negative electrodes, positive electrodes, and separators are immersed in an electrolytic solution, or the separator is impregnated with an electrolytic solution to form a battery.

The organic solvent used in the electrolyte of the lithium secondary battery in the present invention may be any organic solvent as long as it can dissolve the lithium salt.
Preferably, it is an aprotic organic melt having the following properties: high dielectric constant, high stability against redox, wide electrochemical potential window, and low viscosity. It will be done.

Specifically, propylene carbonate, ethylene carbonate, sulfolane, γ-butyrolactone, 1,2-dimethoxyethane,
Tetrahydrofuran, 2methyl-tetrahydrofuran,
Solvents such as acetonitrile, dimethyl sulfoxide, and mixed solvents thereof can be used. In addition, the C degree of the electrolytic solution depends on the type of solvent, electrolyte, and electrode material, but it is 0.
．． A range of 1 to 10 mol/liter is preferred.

Example The present invention will be explained more specifically using the above examples.

Example 1 Pitch fibers with a diameter of 6.2 uLm were spun using pitch containing 95% mesophase as a raw material, and further heated at 2800°C.
Graphitization treatment was performed to obtain carbon fiber. This carbon fiber has a length of 2
A negative electrode was prepared by binding 1 weight of 0 to 30 g of nickel wire with a nickel wire having a diameter of 0.1 layer. The diameter of the carbon fiber after graphitization was approximately 4.5 #Lm.

The crystal parameters of this carbon fiber determined by X-ray diffraction method are:
d(102=0.339ins, Lc=21.
It was 1 nm. The orientation coefficient is 8.2″″, and the size of graphite crystallites in the fiber cross-sectional direction according to the TEM photograph is approximately 45
1! II. For measurements, a 0.1g metal lithium sheet crimped onto a nickel mesh was used as the counter electrode, a small piece of the metal lithium sheet was connected to a nickel wire, similar to the counter electrode, as the reference electrode, and L
A three-electrode cell using a solution of iAsF6 dissolved in propylene carbonate at a concentration of 1.5 mol/liter as an electrolytic solution was used, and the cell was placed in a closed glass container with argon gas as an atmosphere gas. The change in potential of the negative electrode when constant current charging and discharging was repeated was measured using the reference electrode as a reference.

A constant Ti, current charge/discharge cycle test was conducted. charging current,
Both the discharge 'frA and current were set at 0.3 mA. Charging, ie.

The lithium insertion reaction ended when the potential of the test electrode with respect to the reference electrode became O■, the same potential as the lithium metal, and after 30 minutes of open state, discharge, that is, the lithium release reaction started. . The discharging reaction was terminated when the potential of the test electrode with respect to the reference electrode became 1.sup., and after being left open for 30 minutes, the battery was charged again.

The natural potential of the carbon fiber was about 3 . As a result of charging and discharging under these conditions, the discharge efficiency was approximately 80% for the first 11 cycles, but for 5S2 cycles [I and subsequent cycles, 9
It was stable at 6% or more and 1-88% or less. The change in the discharge curve due to repeated charging and discharging is small, and the discharge capacity is 180
It was stable at mAhr/g.

Example 2 Using the same pitch raw material and spinning conditions as Example 1, pitch m1
The fibers were spun to a diameter of 4 gm using Li, and graphitized at 2800°C to obtain carbon fibers. The diameter of carbon fiber after graphitization is approximately 3
It was #Lm.

The crystal parameters of this carbon fiber according to the X-ray diffraction method are d
662 = 0.340in Peng, Lc = 17
．． It was 7 nm. The orientation coefficient is 11.3°, and the size of graphite crystallites in the cross-sectional direction of the fiber according to the TEM photograph is approximately 3
It was 2n.

As a result of testing under the same conditions as in Example 1, the discharge efficiency in the first cycle was 86%, and the discharge efficiency in the second and subsequent cycles was 96% or more and 100% or less. The discharge capacity was 210 mAh/g, and the battery was cycled stably without any decrease in capacity.

FIG. 1 shows cycle changes in the discharge capacity of the carbon fibers used as the negative electrodes in Examples 1 and 2.

Example 3 Using raw material pitch with a menface content of 96%, 40
The fibers were spun using a mesh filter of 0.0 mesh to a diameter of 8.5 gm with pitch fibers, and then heat-treated at 2800°C. The diameter of the carbon fiber after heat treatment was approximately 6.4 m.

The graphitization degree index of this carbon fiber by X-ray diffraction method is d
(, (B = 0.3395m, Lc = 22.I
It was ns. The orientation factor was 9.2°. TEM
The size of the graphite crystallites in the cross-sectional direction of the m-fibers in the photograph was approximately 40 nm.

As a result of testing using a solution of Li (Jj 04 dissolved in propylene carbonate at a concentration of 1.5 mol/liter as the electrolytic solution and using the same conditions as in Example 1, the discharge capacity was 1.
Stable charging and discharging was repeated at 70 mAh/g.

Example 4 Using the same pitch as in Example 3 as a raw material, a pitch fiber with a diameter of 4 g was prepared using a 3000 mesh filter.
The test was conducted using a carbon fiber that was spun into a carbon fiber and heat-treated at 2800° C. as a negative electrode. The diameter of the carbon fibers after heat treatment was about 3 pm.

The graphitization degree index of this carbon fiber by XMA diffraction method is:
d 002 = 3.413A, L C = 133A,
Met.

The orientation coefficient was 17.2@. The size of graphite crystallites in the cross-sectional direction of the fiber according to a TEM photograph was approximately 25 nm. As a result of a charge/discharge test under the same conditions as in Example 1, stable cycling was achieved with a discharge capacity of 180 Ahr/g.

FIG. 2 shows cycle changes in discharge capacity in Examples 3 and 4.

Example 5 Raw pitch with a membrane content of 96% was put into a spinning machine equipped with a stirring rod, and while the stirring rod was rotated at 3° rpm, pitch fibers were spun to a diameter of about 8 wells. Spun into yarn.

Thereafter, heat treatment was performed at 2800°C to form carbon FI1. I got the answer. The diameter of the carbon fibers was approximately 6 lumens.

The graphitization degree index of this carbon fiber by X-ray diffraction method is d
002: 0.3396m+*, L C= 20
．． 8 nm, and the orientation coefficient was 11.2°. The size of the graphite crystallites in the cross-sectional direction of the fiber was approximately 3 Or1M as determined by a TEM photograph.

As a result of a charge/discharge test under the same conditions as in Example 1, stable cycle behavior was exhibited with a discharge capacity of 180 Ahr/g.

Comparative Example 1 Using pitch with a membrane content of 96% as a raw material, peach fiber was spun to a diameter of 15 JLm, and then spun at 2900°C.
A heat-treated carbon am was tested. The diameter of this carbon fiber was approximately 11.4 pm.

The index of graphitization degree by X-ray diffraction method is d002=0-3
379 ni+, L c = 39.7 n rin, and the orientation factor was 5.6°. The size of graphite crystallites in the fiber cross-sectional direction according to a TEM photograph is approximately 180n.
It was m.

As a result of conducting the same test as in Example 1, the initial capacity was 15
The capacity was 0 mAh/g, but as the charge/discharge cycles were repeated, the capacity decreased approximately in proportion to the number of cycles.

FIG. 2 shows Example 3. In conjunction with Example 4, the cycle changes of six discharge cycles are shown.

Effects of the Invention According to Kihatsu IJ], a carbon material suitable for negative electrodes for non-aqueous electrolyte lithium secondary batteries, which has a large discharge rate and high discharge rate stability against repeated charging and discharging. This made it possible to provide the following information:

[Brief explanation of drawings]

FIG. 1 shows the cycle changes between the discharge customers in Example 1 and Example 2. Figure 2 shows Example 3 and Example 4.
This shows the cycle change of six discharges in Comparative Example 1.

Claims (1)

[Claims] A carbon fiber made from mesoface pitch,
The lattice spacing (
d_0_0_2) is 0.338 nm or more 0.343 nm
Below, the crystallite size (Lc) in the c-axis direction is 10 nm or more and 30 nm or less, and the orientation coefficient (FWHM) is 7° or more and 20
° or less, and the average length in the cross-sectional direction of the fiber of graphite crystallites elongated in the fiber axis direction is 20 nm or more 10
A negative electrode for a lithium secondary battery characterized by using carbon fiber having a diameter of 0 nm or less.