JP3840087B2 - Lithium secondary battery and negative electrode material - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery and a negative electrode material having an improved negative electrode configuration.
[0002]
[Prior art]
In recent years, nonaqueous electrolyte batteries using lithium as a negative electrode active material have attracted attention as high energy density batteries, and manganese dioxide (MnO) has been used as a positive electrode active material.₂), Fluorocarbon [(CF₂)_n], Thionyl chloride (SOCl₂) Etc. have already been widely used as a power source for calculators, clocks and memory backup batteries.
[0003]
Furthermore, in recent years, with the reduction in size and weight of various electronic devices such as VTRs and communication devices, the demand for high energy density secondary batteries has increased, and research on lithium secondary batteries using lithium as a negative electrode active material has been made. Is being actively conducted.
[0004]
Lithium secondary batteries use lithium for the negative electrode, and in non-aqueous solvents such as propylene carbonate (PC), 1,2-dimethoxyethane (DME), γ-butyrolactone (γ-BL), and tetrahydrofuran (THF) as the electrolyte. LiClO_Four, LiBF_Four, LiAsF₆A non-aqueous electrolyte or lithium ion conductive solid electrolyte in which a lithium salt such as a lithium salt is dissolved is used, and TiS is mainly used as a positive electrode active material.₂, MoS₂, V₂O_Five, V₆O₁₃, MnO₂It has been studied to use a compound that has a topochemical reaction with lithium.
[0005]
However, the above-described lithium secondary battery has not yet been put into practical use. The main reason is that the charge / discharge efficiency is low and the number of times (cycle life) that can be charged / discharged is short. The cause of this is thought to be largely due to the deterioration of lithium due to the reaction between lithium in the negative electrode and the non-aqueous electrolyte. That is, lithium dissolved in the non-aqueous electrolyte as lithium ions during discharge reacts with the solvent when deposited during charging, and its surface is partially inactivated. For this reason, when charging and discharging are repeated, lithium is deposited in a dendritic shape (dendritic shape) or in a small spherical shape, and further, a phenomenon such that lithium is desorbed from the current collector occurs.
[0006]
For this reason, lithium and non-aqueous electrolysis can be obtained by using a carbonaceous material that occludes / releases lithium, such as coke, resin fired body, carbon fiber, pyrolytic vapor phase carbon, etc., as the negative electrode incorporated in the lithium secondary battery. It has been proposed to improve the deterioration of the negative electrode characteristics due to the reaction with the liquid and the dendrite precipitation.
[0007]
In the negative electrode containing the carbonaceous material, lithium ions enter and exit the portion between the layers in the structure (graphite structure) in which the hexagonal network layers mainly composed of carbon atoms are stacked among the carbonaceous materials. It is thought that charging / discharging is performed. For this reason, it is necessary to use a carbonaceous material having a graphite structure developed to some extent for the negative electrode of the lithium secondary battery. However, when a carbonaceous material obtained by pulverizing a graphitized giant crystal is used as a negative electrode in a non-aqueous electrolyte, the non-aqueous electrolyte is decomposed, resulting in a decrease in battery capacity and charge / discharge efficiency. In particular, when a battery is operated at a high current density, the capacity, charge / discharge efficiency, and voltage drop during discharge become significant. In addition, as the charge / discharge cycle progresses, the crystal structure or fine structure of the carbonaceous material collapses, and the ability to occlude and release lithium deteriorates, resulting in a problem that the cycle life is reduced.
[0008]
In addition, since the powder in the graphitized material is in the form of flakes, the surface of the graphite crystallite into which lithium ions are inserted becomes smaller in the area exposed to the electrolyte, so that it is abrupt in high-rate charge / discharge cycles. There is a problem that capacity decreases. For this reason, although improvement is made by adding carbon black or the like, there arises a problem that the negative electrode filling density is lowered. As a result, high capacity lithium secondary batteries could not be realized with conventional graphitized materials.
[0009]
In addition, graphitized carbon fibers also have problems such as non-aqueous electrolyte decomposition when powdered, and the performance as a negative electrode is significantly reduced, as in the case of using giant crystal powder. Was.
[0010]
On the other hand, in the case of carbonized materials such as coke and carbon fiber with low graphitization degree, the decomposition of the solvent can be suppressed to some extent, but the capacity and charge / discharge efficiency are low, and the charge / discharge overvoltage is large, and the discharge voltage flatness of the battery Are low, and the cycle life is low.
[0011]
Conventionally, they are disclosed in JP-A-62-268058, JP-A-2-82466, JP-A-4-61747, JP-A-4-115458, JP-A-4-184862, JP-A-4-190557, and the like. As described above, the optimum graphitic structure parameters have been proposed by controlling the graphitization degree of various carbonized materials and graphitized materials, but a negative electrode having sufficient characteristics has not been obtained. Moreover, although carbon fiber used as a negative electrode is disclosed by Unexamined-Japanese-Patent No. 4-79170 and Unexamined-Japanese-Patent No. 4-82172, the performance of the negative electrode using the carbonaceous material which powdered it has a problem.
[0012]
[Problems to be solved by the invention]
An object of the present invention is to provide a lithium secondary battery and a negative electrode material having a high capacity and excellent battery characteristics such as charge / discharge efficiency, cycle life, flatness of discharge voltage, and rapid charge / discharge cycle.
[0013]
[Means for Solving the Problems]
  A lithium secondary battery according to the present invention is a lithium secondary battery comprising a positive electrode, a negative electrode containing a carbonaceous material that occludes / releases lithium ions, and a non-aqueous electrolyte.
  The carbonaceous material has an interval d between (002) planes of a graphite structure by X-ray diffraction.₀₀₂Is 0.335 to 0.336The ratio of the rhombohedral (101) diffraction peak by the X-ray diffraction method to the hexagonal (101) diffraction peak by the X-ray diffraction method is 0.6 or less.And (101) diffraction peak P of hexagonal system by X-ray diffraction method ₁₀₁ And (100) diffraction peak P ₁₀₀ Peak intensity ratio (P ₁₀₁ / P ₁₀₀ ) Exceeds 2.2It is characterized by this.
[0014]
  The negative electrode material according to the present invention has an interplanar spacing d of (002) plane of graphite structure by X-ray diffraction method.₀₀₂Is 0.335 to 0.336The ratio of the rhombohedral (101) diffraction peak by the X-ray diffraction method to the hexagonal (101) diffraction peak by the X-ray diffraction method is 0.6 or less.And (101) diffraction peak P of hexagonal system by X-ray diffraction method ₁₀₁ And (100) diffraction peak P ₁₀₀ Peak intensity ratio (P ₁₀₁ / P ₁₀₀ ) Exceeds 2.2It is characterized by containing a carbonaceous material.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a lithium secondary battery (for example, a cylindrical lithium secondary battery) according to the present invention will be described in detail with reference to FIG.
[0016]
The bottomed cylindrical container 1 has an insulator 2 disposed at the bottom. The electrode group 3 is accommodated in the container 1. The electrode group 3 has a structure in which a belt-like material in which a positive electrode 4, a separator 5, and a negative electrode 6 are laminated in this order is wound in a spiral shape so that the negative electrode 6 is located outside.
[0017]
An electrolytic solution is accommodated in the container 1. The insulating paper 7 having a central opening is placed above the electrode group 3 in the container 1. The insulating sealing plate 8 is disposed in the upper opening of the container 1, and the sealing plate 8 is liquid-tightly fixed to the container 1 by caulking the vicinity of the upper opening inward. The positive terminal 9 is fitted in the center of the insulating sealing plate 8. One end of the positive electrode lead 10 is connected to the positive electrode 4, and the other end is connected to the positive electrode terminal 9. The negative electrode 6 is connected to the container 1 serving as a negative electrode terminal through a negative electrode lead (not shown).
[0018]
The container 1 is made of, for example, stainless steel.
[0019]
The positive electrode 4 is produced by suspending a conductive agent and a binder in an appropriate solvent in a positive electrode active material, and applying the suspension to a current collector and drying to form a thin plate.
[0020]
Examples of the positive electrode active material include various oxides such as manganese dioxide, lithium manganese composite oxide, lithium-containing nickel oxide, lithium-containing cobalt compound, lithium-containing nickel cobalt oxide, vanadium oxide containing lithium, and disulfide. Examples thereof include chalcogen compounds such as titanium and molybdenum disulfide. Among them, lithium cobalt oxide (LiCoO₂), Lithium nickel oxide (LiNiO)₂), Lithium manganese oxide (LiMn)₂O_FourLiMnO₂) Is preferable because a high voltage can be obtained.
[0021]
Examples of the conductive agent include acetylene black, carbon black, and graphite.
[0022]
Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDE), ethylene-propylene-diene copolymer (EPDM), and styrene-butadiene rubber (SBR).
[0023]
The mixing ratio of the positive electrode active material, the conductive agent and the binder is preferably in the range of 80 to 95% by weight of the positive electrode active material, 3 to 20% by weight of the conductive agent, and 2 to 7% by weight of the binder.
[0024]
As the current collector, for example, aluminum foil, stainless steel foil, nickel foil or the like can be used.
[0025]
As the separator 5, for example, a synthetic resin nonwoven fabric, a polyethylene porous film, a polypropylene porous film, or the like can be used.
[0026]
The negative electrode 6 includes a carbonaceous material described in the following (1) to (4). However, La, d for determining carbonaceous material₀₀₂, Lc and intensity ratio (P₁₀₁/ P₁₀₀) Measurement and definition are as follows.
[0027]
(A) All data measured by the X-ray diffraction method used CuKα as an X-ray source and high-purity silicon as a standard substance. La, d₀₀₂, Lc was determined from the position of each diffraction peak and the half-value width. As the calculation method, the half-width half-point method was used.
[0028]
(B) The crystallite length La in the a-axis direction and the crystallite length Lc in the c-axis direction are values when K, which is the shape factor of Scherrer's formula, is 0.89.
[0029]
(C) (101) diffraction peak P by X-ray diffraction method₁₀₁And (100) diffraction peak P₁₀₀Intensity ratio (P₁₀₁/ P₁₀₀) Is obtained from the height ratio of these peaks.
[0030]
(1) Carbonaceous material
This carbonaceous material has an interplanar spacing d of the (002) plane of the graphite structure by X-ray diffraction.₀₀₂Is in the range of 0.335 to 0.336 nm and the (101) diffraction peak P₁₀₁And (100) diffraction peak P₁₀₀Peak intensity ratio (P₁₀₁/ P₁₀₀) Exceeds 2.2, and the ratio (La / Lc) of the crystallite length La in the a-axis direction to the crystallite length Lc in the c-axis direction is less than 1.5.
[0031]
The carbonaceous material is obtained by an X-ray diffraction method and has an average surface spacing d of (002) planes.₀₀₂Is 0.335 to 0.336 nm. In such a carbonaceous material, the number of sites into which lithium ions can be inserted increases, and the capacity increases. However, in the carbonaceous material having an average surface spacing that deviates from the above range, the capacity is lowered, and the flatness of the voltage when the battery is discharged is lowered.
[0032]
The intensity ratio (P₁₀₁/ P₁₀₀) Exceeds 2.2, the graphite structure is very developed. Such a graphite structure has few misalignments, twists, and angles between stacked hexagonal mesh layers. Since this has the characteristics of conventional natural graphite, it causes a decrease in capacity under high-rate charge / discharge conditions. Therefore, by setting the ratio (La / Lc) of the crystallite length La in the a-axis direction to the crystallite length Lc in the C-axis direction by the X-ray diffraction method to be less than 1.5, the high-rate A graphitized product avoiding a decrease in capacity under charge / discharge conditions could be obtained. That is, by making the La shorter and the Lc longer than the conventional graphite, the lithium ion insertion / desorption reaction can be carried out smoothly, and the problems of the layer structure can be overcome. A high capacity can be obtained under the following charge / discharge conditions.
[0033]
The carbonaceous material desirably has a crystallite length La in the a-axis direction of a graphite structure of 20 to 100 nm, more preferably 40 to 80 nm, based on a diffraction peak of (110) plane obtained by X-axis diffraction. Moreover, it is preferable that the shape of the a-axis surface in the graphite structure of the carbonaceous material is a rectangle.
[0034]
The carbonaceous material in which La is in the above range has a moderately developed graphite structure and an appropriate length of the crystallite in the a-axis direction, so that lithium ions diffuse between the layers of the hexagonal network layer. In addition, the number of sites where lithium ions enter and exit increases, and the lithium ion can be occluded and released.
[0035]
The carbonaceous material whose La exceeds 100 nm becomes a giant crystal, and it becomes difficult for lithium ions to diffuse between the layers of the hexagonal network layer, so that it is difficult to ensure the amount of occlusion / release of lithium ions. Moreover, the surface is active with respect to a nonaqueous solvent, and this solvent becomes easy to carry out reductive decomposition. On the other hand, since the carbonaceous material with La less than 20 nm contains a large amount of carbonaceous material with an undeveloped graphite structure in the carbonaceous material, reversible occlusion / release of lithium ions between hexagonal network layers is small. There is a fear.
[0036]
The carbonaceous material desirably has a c-axis direction crystallite size Lc of 15 nm or more, more preferably 20 to 100 nm, obtained by X-ray diffraction. A carbonaceous material having Lc in such a range has a property that a graphite structure is moderately developed and a large amount of lithium ions are reversibly occluded / released.
[0037]
As a measure of the ratio of the graphite structure and the disordered layer structure constituting the carbonaceous material, there is a Raman spectrum of the carbonaceous material measured using an argon laser (wavelength 514.5 nm) as a light source. The Raman spectrum measured for the carbonaceous material is 1360 cm.^-1A peak derived from the turbulent structure that appears in the vicinity, and 1580 cm^-1There is a peak derived from the graphite structure that appears in the vicinity. The peak intensity ratio, that is, 1580 cm in the argon laser Raman spectrum (wavelength 514.5 nm).^-11360cm against the peak intensity (R2)^-1It is preferable to use a carbonaceous material having a peak intensity (R1) ratio (R1 / R2) value of 0.2 or less.
[0038]
The carbonaceous material has a true density of 2.20 g / cm, which is a measure of the degree of graphitization.^ThreeThe above is preferable.
[0039]
It is preferable that 90% by volume or more of the carbonaceous material has a particle size distribution of 1 to 100 μm and an average particle size of 1 to 80 μm. N₂Specific surface area of 0.1-40m by BET method of gas adsorption²/ G is preferable. The carbonaceous material having such a particle size distribution and specific surface area can improve the packing density of the negative electrode, and at the same time reduce the content of carbonaceous material or amorphous carbon having a broken graphite crystal structure that is active against non-aqueous solvents, Reductive decomposition of the nonaqueous solvent can be suppressed. Further, a carbonaceous material containing particles having a particle size of 0.5 μm or less in a particle size distribution of 5% by volume or less further reduces carbonaceous material having a collapsed graphite crystal structure or amorphous carbon, and effectively reduces reductive decomposition of a non-aqueous solvent. It becomes possible to suppress. Conversely, the particle size distribution and N₂If the specific surface area by the BET method of gas adsorption becomes too large, the content of amorphous carbonaceous material and the content of carbonaceous material with a broken graphite crystal structure will increase, and the reductive decomposition of the solvent will easily occur. Decreases.
[0040]
The carbonaceous material preferably has a sulfur content of 1000 ppm or less (including 0 ppm). Such a carbonaceous material having a low sulfur content increases the amount of occlusion / release of lithium ions and reduces the reductive decomposition of the nonaqueous solvent.
[0041]
In a carbonaceous material having a graphite structure developed to some extent, if the sulfur content is set to a low value of 1000 ppm or less (including 0 ppm), there are few defects in the graphite structure, and as a result, the graphite structure is less likely to collapse. It is thought that the amount of occlusion / release increases. For this reason, the capacity | capacitance, charging / discharging efficiency, and cycle life of a lithium secondary battery can be improved. Moreover, it is thought that decomposition | disassembly of the nonaqueous solvent and reaction of an electrode reaction by reaction with a nonaqueous solvent and lithium ion, and sulfur or a sulfur compound are reduced, and it can improve charging / discharging efficiency and cycle life. That is, when sulfur in the carbonaceous material reacts with lithium ions, a stable group such as LiS- or a compound such as LiS is produced. The lithium is thought not to contribute to the reversible storage / release reaction. In addition, it is considered that the resulting compound becomes an obstacle between the hexagonal network layers, preventing the smooth insertion of lithium ions. These factors reduce charge / discharge efficiency and cycle life.
[0042]
As other impurities in the carbonaceous material, oxygen, nitrogen, silicon, or metal elements such as Fe and Ni are preferably as few as possible. Specifically, the oxygen content is preferably 500 ppm or less, the nitrogen content is 1000 ppm or less, and the metal elements such as Fe and Ni are each preferably 50 ppm or less. If these impurity elements exceed the above range, lithium ions between the carbon layers may be consumed by reacting with the impurity elements. However, aluminum, boron, phosphorus, calcium, tin, etc. are allowed to be contained.
[0043]
Next, an example of a method for producing the carbonaceous material will be described.
[0044]
First, a short fiber having a fiber length of 200 to 300 μm is spun using a mesophase pitch as a raw material by a melt blow method, and then carbonized to such an extent that it can be infusibilized and pulverized. The carbonization heat treatment is desirably performed at 600 to 2000 ° C., preferably 800 to 1500 ° C. Surface spacing d of (002) plane of carbonized mesophase pitch carbon fiber by X-ray diffraction method₀₀₂Is preferably 0.344 nm or more, more preferably 0.357 nm or more. Such a carbon fiber is suitable for pulverization because it has a fine structure suitable for shortening the fiber length. In contrast, the surface spacing d₀₀₂When carbon fibers having a diameter of less than 0.344 nm are pulverized, vertical cracks occur and it is difficult to shorten the fiber length. Next, the carbonaceous material described above is produced by graphitizing the carbonized and pulverized carbon fibers at 2000 ° C. or higher, more preferably 2500 to 3200 ° C. At this time, the pulverization and firing steps are extremely important, and a spherical, granular, block, or cylindrical shape is graphitized by pulverizing for an appropriate time (within 20 hours) using a ball mill or a jet mill at the time of pulverization. It is preferable to do. Moreover, it is necessary to prevent the crystal having a rhombohedral system in the crystal structure of the graphitized product from exceeding 30%. This is because the hexagonal graphite structure is broken and a rhombohedral structure is formed by pulverization, but it is preferably 0 to 30% by volume by appropriate pulverization. If this range is exceeded, decomposition of the electrolyte solution occurs, so the negative electrode performance decreases.
[0045]
The ratio of La to Lc and the crystal structure of the carbonaceous material produced by the above method can be controlled by defining the grinding conditions (grinding time, atmosphere, grinding power).
[0046]
When producing a carbonaceous material by the above-described method, it is desirable that the particle size distribution and specific surface area of the obtained carbonaceous material have the values described above.
[0047]
The carbonaceous material can be obtained by graphitizing coke or bulk mesophase obtained using, for example, low-sulfur petroleum pitch, coal tar and the like at 2500 to 3200 ° C. in addition to those obtained by the above-described method.
[0048]
  In addition,Interplanar spacing d of (002) plane of graphite structure by X-ray diffraction method ₀₀₂ Is in the range of 0.335 to 0.336 nm, and (101) diffraction peak P of hexagonal system by X-ray diffraction method ₁₀₁ And (100) diffraction peak P ₁₀₀ Peak intensity ratio (P ₁₀₁ / P ₁₀₀ ) Exceeds 2.2The carbonaceous material can be defined such that the ratio of the rhombohedral system to the hexagonal system is such that the ratio of the (101) diffraction peak by the X-ray diffraction method is 0.6 or less. A carbonaceous material having a rhombohedral structure in such a ratio can increase the negative electrode charge / discharge efficiency.
[0049]
(2) Carbonaceous material
This carbonaceous material has an interplanar spacing d of the (002) plane of the graphite structure in X-ray diffraction.₀₀₂Is a diffraction peak derived from 0.340 nm or less and the surface spacing d₀₀₂Is a powder in which a diffraction peak derived from the range of 0.344 to 0.370 nm appears.
[0050]
The carbonaceous material has a (002) plane spacing d.₀₀₂Between the graphitized material having a huge graphite crystal (having a thickness La in the a-axis direction of 20 nm or more) and a (002) plane distance d.₀₀₂Is made of a small carbonized graphite crystal (Lc of 6 nm or less) showing a diffraction peak derived from the range of 0.344 to 0.370 nm, so that high capacity can be maintained with little cycle deterioration even in rapid charging. It becomes possible. The latter (002) diffraction peak is more preferably a carbonized product derived from the range of 0.356 to 0.370 nm.
[0051]
Conventionally, (002) plane spacing d₀₀₂In the negative electrode made of the graphitized material exhibiting a diffraction peak derived from the range of 0.340 nm or less, the cycle deterioration is large and the capacity is small during rapid charging. Also, the (002) plane spacing d₀₀₂The carbonaceous material exhibiting a diffraction peak derived from the range of 0.344 to 0.370 nm has a true density of 1.70 to 2.15 / cm.^ThreeTherefore, the capacity per unit volume of the negative electrode (mAh / cm) is smaller than that of the graphitized material.^Three) Has the problem of becoming smaller.
[0052]
In order to improve such problems, the carbonaceous material constituting the negative electrode of the present invention has both the graphitized material and the carbonized material having the characteristics described above, so that the rapid charge characteristics and capacity can be greatly improved. It became. This is a fast charge at a constant current (2 mA / cm²Above) Sometimes the (002) plane spacing d₀₀₂Is selectively inserted into a graphite crystal exhibiting a diffraction peak derived from the range of 0.344 to 0.370 nm. At this time, the diffusion rate of lithium ions is the distance d of the (002) plane.₀₀₂Is very fast compared to graphitized material showing a diffraction peak derived from 0.340 nm or less, and thus can be rapidly charged. Further, when the quick charge progresses, the charge end voltage is reached, and when switching to the constant voltage charge at that voltage, the surface spacing d of the (002) plane is mainly used.₀₀₂Lithium ions are selectively inserted into a graphite crystal exhibiting a diffraction peak derived from 0.340 nm or less at a moderate rate. Such a selective insertion reaction of lithium ions enables rapid charging and maintains a high capacity.
[0053]
As the carbonaceous material, for example, the spacing d of the (002) plane of the graphite structure in X-ray diffraction.₀₀₂Is a multiphase graphitized product in which a diffraction peak derived from 0.340 nm or less and a diffraction peak derived from the range of 0.344 to 0.370 nm appear, or the (002) plane spacing d₀₀₂Between the graphitized material showing a diffraction peak derived from 0.340 nm or less and the (002) plane spacing d₀₀₂Can be a mixture of carbonized compounds showing a diffraction peak derived from the range of 0.344 nm to 0.370 nm.
[0054]
Examples of the multi-phase graphitized material include poly-vinylidene chloride, phenol resin, furfural resin, charcoal, anthracite, sugar, pitch, etc., by performing non-uniform graphitization at a temperature of 1500 to 3000 ° C. under normal pressure or high pressure. It is obtained by phase graphitization. By performing graphitization in a non-uniform manner in this way, the surface spacing d of the (002) planes.₀₀₂Between the graphitized phase showing a diffraction peak derived from 0.340 nm or less and the (002) plane spacing d₀₀₂Can be obtained by coexisting with a low graphitized phase exhibiting a diffraction peak derived from 0.344 nm to 0.370 nm.
[0055]
Examples of the graphitized material as one component of the mixture include powders such as graphite, anisotropic pitch-based carbon fiber, spherical carbon, pyrolytic vapor-grown carbon, and coke. The graphitized material has a specific surface area of 0.5 to 10 m.²More preferred are graphite, mesophase pitch-based carbon fiber powder, and mesophase microspheres in the range of / g.
[0056]
Examples of the carbonized material as the other component of the mixture include low sulfur content (2000 ppm or less), low nitrogen content (1000 ppm or less) coke, high-purity anisotropic pitch-based carbon fiber, spherical carbon, and pyrolysis gas phase. Examples thereof include powders such as a grown carbon body and a fired resin body. The carbonized product has a specific surface area of 2 to 30 m.²More preferred are carbon fiber powder, high-purity mesophase pitch-based carbon fiber powder, mesophase microspheres, and low-sulfur-containing coke using synthetic pitches (such as naphthalene) in the range of / g. Since any of the carbonized products has a firing temperature in the range of 600 to 1200 ° C., it is important to reduce impurities such as sulfur and transition metals (iron, nickel) in the carbonized product. For this reason, it is necessary to select a material having a small amount of the impurities in the raw material petroleum pitch, mesophase pitch, and coal tar.
[0057]
(3) Carbonaceous material
This carbonaceous material is a graphitized material mainly composed of a block shape, a fiber shape, or a spherical shape, and has an interval d between (002) planes of the graphite structure by X-ray diffraction.₀₀₂Is in the range of 0.335 to 0.337 nm and the (101) diffraction peak P₁₀₁And (100) diffraction peak P₁₀₀Peak intensity ratio (P₁₀₁/ P₁₀₀) Has a property exceeding 2.2.
[0058]
The carbonaceous material is obtained by an X-ray diffraction method and has an average surface spacing d of (002) planes.₀₀₂Is 0.335 to 0.337 nm. In such a carbonaceous material, the number of sites into which lithium ions can be inserted increases, and the capacity increases. However, in the carbonaceous material having an average surface spacing that deviates from the above range, the capacity is lowered, and the flatness of the voltage when the battery is discharged is lowered.
[0059]
The intensity ratio (P₁₀₁/ P₁₀₀) Exceeds 2.2, the graphite structure is very developed. Such a graphite structure has few misalignments, twists, and angles between stacked hexagonal mesh layers. Since this has the characteristics of conventional natural graphite, there is a risk that the capacity will decrease under high-rate charge / discharge conditions. Therefore, the ratio (La / Lc) of the crystallite length La in the a-axis direction to the crystallite length Lc in the C-axis direction by the X-ray diffraction method is preferably less than 1.5. By defining La / Lc in this way, it is possible to obtain a graphitized product that avoids a decrease in capacity under the high-rate charge / discharge conditions. That is, by making the La shorter and the Lc longer than the conventional graphite, the lithium ion insertion / desorption reaction can be carried out smoothly, and the problems of the layer structure can be overcome. A high capacity can be obtained under the following charge / discharge conditions.
[0060]
The carbonaceous material has a block shape, a fiber shape, or a spherical shape. Here, the block shape means particles other than flakes, and the ratio of the major axis to the minor axis (major axis / minor axis) is 5 or less, more preferably 2 or less. The fiber shape has a ratio of fiber length to fiber diameter (fiber length / fiber diameter) in the range of 0.5 to 10, and the orientation of crystallites is radial in the cross section perpendicular to the fiber length direction. Preferably, it is in the form of a strip or Brook Stellar. The spherical shape is a sphere having a major axis / minor axis ratio (major axis / minor axis) in the range of 1 to 2, and the orientation of the crystallites is preferably radial, lamellar, or Brooke Stellar in its cross section. Such a carbonaceous material having a block shape, a fiber shape, or a spherical shape can increase the exposed area in the c-axis direction, so that a high-capacity secondary battery can be obtained under high-rate charge / discharge conditions. . In particular, a carbonaceous material having a fibrous or spherical shape in which the orientation of crystallites is radial, lamellar, or Brook Stellar type can further increase the exposed area in the c-axis direction, and can perform lithium ion insertion / desorption reaction. It becomes possible to perform smoothly.
[0061]
In the carbonaceous material, the length La of the crystallite in the a-axis direction of the graphite structure by the diffraction peak of the (110) plane obtained by the X-ray diffraction method is 20 to 100 nm, more preferably 40 to 80 nm. Moreover, it is preferable that the shape of the a-axis surface in the graphite structure of the carbonaceous material is a rectangle.
[0062]
The carbonaceous material in which La is in the above range has a moderately developed graphite structure and an appropriate length of the crystallite in the a-axis direction, so that lithium ions diffuse between the layers of the hexagonal network layer. In addition, the number of sites where lithium ions enter and exit increases, and the lithium ion can be occluded and released.
[0063]
The carbonaceous material whose La exceeds 100 nm becomes a giant crystal, and it becomes difficult for lithium ions to diffuse between the layers of the hexagonal network layer, so that it is difficult to ensure the amount of occlusion / release of lithium ions. Moreover, the surface is active with respect to a nonaqueous solvent, and this solvent becomes easy to carry out reductive decomposition. On the other hand, since the carbonaceous material with La less than 20 nm contains a large amount of carbonaceous material with an undeveloped graphite structure in the carbonaceous material, reversible occlusion / release of lithium ions between hexagonal network layers is small. There is a fear.
[0064]
The carbonaceous material desirably has a crystallite size Lc in the c-axis direction obtained by X-ray diffraction of 15 nm or more, more preferably 25 to 200 nm. A carbonaceous material having Lc in such a range has a property that a graphite structure is moderately developed and a large amount of lithium ions are reversibly occluded / released.
[0065]
As a measure of the ratio of the graphite structure and the disordered layer structure constituting the carbonaceous material, there is a Raman spectrum of the carbonaceous material measured using an argon laser (wavelength 514.5 nm) as a light source. The Raman spectrum measured for the carbonaceous material is 1360 cm.^-1A peak derived from the turbulent structure that appears in the vicinity, and 1580 cm^-1There is a peak derived from the graphite structure that appears in the vicinity. The peak intensity ratio, that is, 1580 cm in the argon laser Raman spectrum (wavelength 514.5 nm).^-11360cm against the peak intensity (R2)^-1It is preferable to use a carbonaceous material having a peak intensity (R1) ratio (R1 / R2) value of 0.2 or less.
[0066]
The carbonaceous material has a true density of 2.20 g / cm, which is a measure of the degree of graphitization.^ThreeThe above is preferable.
[0067]
It is preferable that 90% by volume or more of the carbonaceous material has a particle size distribution of 1 to 100 μm and an average particle size of 1 to 80 μm. N₂Specific surface area by gas adsorption BET method is 0.1-10m²/ G is preferable. The carbonaceous material having such a particle size distribution and specific surface area can improve the packing density of the negative electrode, and at the same time reduce the content of carbonaceous material or amorphous carbon having a broken graphite crystal structure that is active against non-aqueous solvents, Reductive decomposition of the nonaqueous solvent can be suppressed. Further, a carbonaceous material containing particles having a particle size of 0.5 μm or less in a particle size distribution of 5% by volume or less further reduces carbonaceous material having a collapsed graphite crystal structure or amorphous carbon, and effectively reduces reductive decomposition of a non-aqueous solvent. It becomes possible to suppress. Conversely, the particle size distribution and N₂If the specific surface area by the BET method of gas adsorption becomes too large, the content of amorphous carbonaceous material and the content of carbonaceous material with a broken graphite crystal structure will increase, and the reductive decomposition of the solvent will easily occur. Decreases.
[0068]
The carbonaceous material preferably has a sulfur content of 1000 ppm or less (including 0 ppm). Such a carbonaceous material having a low sulfur content increases the amount of occlusion / release of lithium ions and reduces the reductive decomposition of the nonaqueous solvent.
[0069]
In a carbonaceous material having a graphite structure developed to some extent, if the sulfur content is set to a low value of 1000 ppm or less (including 0 ppm), there are few defects in the graphite structure, and as a result, the graphite structure is less likely to collapse. It is thought that the amount of occlusion / release increases. For this reason, the capacity | capacitance, charging / discharging efficiency, and cycle life of a lithium secondary battery can be improved. Moreover, it is thought that decomposition | disassembly of the nonaqueous solvent and reaction of an electrode reaction by reaction with a nonaqueous solvent and lithium ion, and sulfur or a sulfur compound are reduced, and it can improve charging / discharging efficiency and cycle life. That is, when sulfur in the carbonaceous material reacts with lithium ions, a stable group such as LiS- or a compound such as LiS is produced. The lithium is thought not to contribute to the reversible storage / release reaction. In addition, it is considered that the resulting compound becomes an obstacle between the hexagonal network layers, preventing the smooth insertion of lithium ions. These factors reduce charge / discharge efficiency and cycle life.
[0070]
As other impurities in the carbonaceous material, oxygen, nitrogen, silicon, or metal elements such as Fe and Ni are preferably as few as possible. Specifically, the oxygen content is preferably 500 ppm or less, the nitrogen content is 1000 ppm or less, and the metal elements such as Fe and Ni are each preferably 50 ppm or less. If these impurity elements exceed the above range, lithium ions between the carbon layers may be consumed by reacting with the impurity elements. However, aluminum, boron, phosphorus, calcium, tin, etc. are allowed to be contained.
[0071]
Next, an example of a method for producing the carbonaceous material will be described.
[0072]
First, a short fiber having a fiber length of 200 to 300 μm is spun using a mesophase pitch as a raw material by a melt blow method, and then carbonized to such an extent that it can be infusibilized and pulverized. The conditions for infusibilization at this time greatly affect graphitization, which will be described later, and it is desirable to insolubilize slowly. The carbonization heat treatment is desirably performed at 600 to 2000 ° C, preferably 800 to 1500 ° C. Surface spacing d of (002) plane of carbonized mesophase pitch carbon fiber by X-ray diffraction method₀₀₂Is preferably 0.344 nm or more, more preferably 0.357 nm or more. Such a carbon fiber is suitable for pulverization because it has a fine structure suitable for shortening the fiber length. In contrast, the surface spacing d₀₀₂When carbon fibers having a diameter of less than 0.344 nm are pulverized, vertical cracks occur and it is difficult to shorten the fiber length. Next, the carbonaceous material described above is produced by graphitizing the carbonized and pulverized carbon fibers at 2000 ° C. or higher, more preferably 2500 to 3200 ° C. At this time, the pulverization and firing steps are extremely important, and the spheres, blocks, and fibers are graphitized by pulverizing for an appropriate time (within 20 hours) using a ball mill or a jet mill at the time of pulverization. is required. Further, it may be a mixture containing at least two kinds of graphitized materials, spherical, block-shaped and fibrous. Moreover, it is necessary to prevent the crystal having a rhombohedral system in the crystal structure of the graphitized product from exceeding 30%. This is because the hexagonal graphite structure is broken and a rhombohedral structure is formed by pulverization, but it is preferably 0 to 30% by volume by appropriate pulverization. If this range is exceeded, decomposition of the electrolyte solution occurs, so the negative electrode performance decreases.
[0073]
The ratio of La to Lc and the crystal structure of the carbonaceous material produced by the above method can be controlled by defining the grinding conditions (grinding time, atmosphere, grinding power).
[0074]
When producing a carbonaceous material by the above-described method, it is desirable that the particle size distribution and specific surface area of the obtained carbonaceous material have the values described above.
[0075]
The carbonaceous material is obtained by graphitizing coke, mesophase spherules, or bulk mesophase obtained using, for example, low sulfur petroleum pitch, coal tar, etc., at 2500 to 3200 ° C. in addition to those obtained by the above-described method. be able to.
[0076]
  In addition,Interplanar spacing d of (002) plane of graphite structure by X-ray diffraction method ₀₀₂ Is in the range of 0.335 to 0.336 nm, and (101) diffraction peak P of hexagonal system by X-ray diffraction method ₁₀₁ And (100) diffraction peak P ₁₀₀ Peak intensity ratio (P ₁₀₁ / P ₁₀₀ ) Exceeds 2.2The carbonaceous material can be defined such that the ratio of the rhombohedral system to the hexagonal system is such that the ratio of the (101) diffraction peak by the X-ray diffraction method is 0.6 or less. A carbonaceous material having a rhombohedral structure in such a ratio can increase the negative electrode charge / discharge efficiency.
[0077]
(4) Carbonaceous material
This carbonaceous material has an interplanar spacing d of the (002) plane of the graphite structure in X-ray diffraction.₀₀₂Between the graphitized material showing a diffraction peak derived from 0.3370 nm or less and the (002) plane spacing d of the graphite structure in X-ray diffraction₀₀₂Is graphitized which shows a diffraction peak derived from a range exceeding 0.3370 nm and not exceeding 0.340 nm.
[0078]
The carbonaceous material has a (002) plane spacing d.₀₀₂Between the graphitized material having a huge graphite crystal (thickness La in the a-axis direction of 40 nm or more) and a (002) plane distance d₀₀₂Is composed of a relatively small graphitized crystal (La is 40 nm or less) showing a diffraction peak derived from the range exceeding 0.3370 nm and 0.340 nm or less, so that there is little cycle deterioration even in rapid charging. A high capacity can be maintained. The latter graphitized material preferably has a (002) diffraction peak in the range of 0.3370 to 0.3380 nm.
[0079]
(002) surface spacing d₀₀₂In the negative electrode made of the graphitized material showing a diffraction peak derived from the range of 0.3370 nm or less, the cycle deterioration is large and the capacity is also small in rapid charging. Also, the (002) plane spacing d₀₀₂Has a capacity of 200 to 280 mAh / cm. The negative electrode made of graphitized material showing a diffraction peak exceeding 0.3370 nm and having a diffraction peak derived from the range of 0.340 nm.^ThreeAnd has a problem that the capacity is smaller than that of the graphitized material.
[0080]
In order to improve such problems, the carbonaceous material constituting the negative electrode of the present invention has two types of graphitized materials having the above-mentioned characteristics, so that quick charge characteristics and capacity can be greatly improved. It was. This is a fast charge at a constant current (2 mA / cm²Above) Sometimes the (002) plane spacing d₀₀₂Lithium ions are selectively inserted into a graphite crystal that exhibits a diffraction peak derived from a range exceeding 0.3370 nm and not exceeding 0.340 nm. At this time, the diffusion rate of lithium ions is the distance d of the (002) plane.₀₀₂Is very fast compared to the graphitized material showing a diffraction peak derived from 0.3370 nm or less, and thus can be rapidly charged. Further, when the quick charge progresses, the charge end voltage is reached, and when switching to the constant voltage charge at that voltage, the surface spacing d of the (002) plane is mainly used.₀₀₂Lithium ions are selectively inserted into the graphite crystal exhibiting a diffraction peak derived from 0.3370 nm or less at a moderate rate. Such a selective insertion reaction of lithium ions enables rapid charging and maintains a high capacity.
[0081]
As the carbonaceous material, for example, the spacing d of the (002) plane of the graphite structure in X-ray diffraction.₀₀₂Is a multiphase graphitized material comprising a graphitized material having a diffraction peak derived from 0.3370 nm or less and a graphitized material having a diffraction peak exceeding 0.3370 nm and derived from a range of 0.340 nm or less, or (002) plane Surface spacing d₀₀₂Between the graphitized material showing a diffraction peak derived from 0.3370 nm or less and the (002) plane spacing d₀₀₂May include a mixture of graphitized materials exhibiting a diffraction peak derived from a range exceeding 0.3370 nm and not exceeding 0.340 nm.
[0082]
Examples of the multi-phase graphitized material include poly-vinylidene chloride, phenol resin, furfural resin, charcoal, anthracite, sugar, pitch, etc., by performing non-uniform graphitization at a temperature of 1500 to 3000 ° C. under normal pressure or high pressure. It is obtained by phase graphitization. By performing graphitization in a non-uniform manner in this way, the surface spacing d of the (002) planes.₀₀₂Is a graphitized phase exhibiting a diffraction peak derived from 0.3370 nm or less and the (002) plane spacing d₀₀₂Having a low graphitization phase exhibiting a diffraction peak derived from the range of more than 0.3370 nm and 0.340 nm or less can be obtained.
[0083]
The surface spacing d of the (002) plane which is one component of the mixture.₀₀₂Examples of graphitized materials having a diffraction peak derived from 0.3370 nm or less include graphite, anisotropic pitch-based carbon fiber graphitized at 2300 to 3200 ° C., spherical carbon, pyrolytic vapor-grown carbon body, coke, etc. A powder is mentioned. The graphitized material has a specific surface area of 2 to 10 m.²/ G, graphite having an average particle diameter in the range of 1 to 30 μm, mesophase pitch-based carbon fiber powder, and mesophase microspheres are more preferable.
[0084]
Spacing d of (002) plane, which is the other component of the mixture₀₀₂Graphitized products exhibiting diffraction peaks exceeding 0.3370 nm and not exceeding 0.340 nm include, for example, low sulfur content (2000 ppm or less), low nitrogen content (1000 ppm or less) coke, high purity anisotropic pitch Examples thereof include powders such as carbon fiber, spherical carbon, pyrolytic vapor growth carbon body, and resin fired body. The graphitized material has a specific surface area of 1 to 10 m.²More preferred are carbon fiber powder, high-purity mesophase pitch-based carbon fiber powder, mesophase microspheres, and low-sulfur-containing coke using synthetic pitches (such as naphthalene) in the range of / g. Any of the graphitized materials can be obtained by firing the raw materials at a temperature in the range of 2300 to 3000 ° C.
[0085]
The distance (d) between the (002) planes₀₀₂Is preferably 5 to 30% by weight of the graphitized material showing a diffraction peak derived from 0.3370 nm or less. If the mixing weight ratio exceeds 30% by weight, the rate characteristics may deteriorate. On the other hand, if the mixing weight ratio is less than 5% by weight, the battery capacity may be reduced.
[0086]
The negative electrode 6 containing the carbonaceous material described in the above (1) to (4) is specifically produced by the following method. A binder is suspended in the carbonaceous material in a suitable solvent, and the suspension is applied to a current collector and dried to form a thin plate, thereby producing the positive electrode.
[0087]
Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC), and the like. Can be used.
[0088]
The blending ratio of the carbonaceous material and the binder is preferably in the range of 90 to 98% by weight of the carbon material and 2 to 10% by weight of the binder. In particular, the carbonaceous material is 5 to 20 mg / cm in a state where the negative electrode 6 is produced.²It is preferable to be in the range.
[0089]
As the current collector, for example, copper foil, stainless steel foil, nickel foil or the like can be used.
[0090]
The nonaqueous electrolytic solution accommodated in the container 1 is prepared by dissolving an electrolyte in a nonaqueous solvent.
[0091]
As the non-aqueous solvent, a known non-aqueous solvent can be used as a solvent for a lithium secondary battery, and is not particularly limited. However, ethylene carbonate (EC) has a lower melting point than ethylene carbonate and has 18 donors. It is preferable to use a nonaqueous solvent mainly composed of a mixed solvent of one or more of the following nonaqueous solvents (hereinafter referred to as a second solvent). Such a non-aqueous solvent is advantageous in that it is stable with respect to a carbonaceous material having a developed graphite structure that constitutes the negative electrode, reductive decomposition or oxidative decomposition of the electrolytic solution does not easily occur, and has high conductivity.
[0092]
Non-aqueous electrolyte containing ethylene carbonate alone has the advantage that it is difficult to be reduced and decomposed with respect to graphitized carbonaceous material, but has a high melting point (39 ° C. to 40 ° C.) and a high viscosity, so the conductivity is small. It is not suitable for secondary batteries operating at room temperature. The second solvent mixed with ethylene carbonate improves the conductivity by making the mixed solvent less viscous than the ethylene carbonate. Further, by using a second solvent having a donor number of 18 or less (provided that the number of donors of ethylene carbonate is 16.4), the ethylene carbonate is not easily selectively solvated to lithium ions, and a graphite structure has been developed. It is considered that the reduction reaction of the second solvent is suppressed with respect to the carbonaceous material. Further, by setting the number of donors of the second solvent to 18 or less, the oxidative decomposition potential easily becomes 4 V or more with respect to the lithium electrode, and there is an advantage that a high voltage lithium secondary battery can be realized.
[0093]
Examples of the second type solvent include dimethyl carbonate (DMC), diethyl carbonate (DEC), propylene carbonate (PC), γ-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA), toluene, Xylene, methyl acetate (MA), etc. are mentioned. These second solvents can be used alone or in the form of a mixture of two or more. In particular, the second type solvent preferably has a donor number of 16.5 or less.
[0094]
The viscosity of the second solvent is preferably 28 mp or less at 25 ° C. The blending amount of the ethylene carbonate in the mixed solvent is preferably 10 to 80% by volume ratio. If it deviates from this range, there is a possibility that the conductivity is lowered or the solvent is decomposed, and the charge / discharge efficiency is lowered. The blending amount of the ethylene carbonate is more preferably 20 to 75% by volume ratio. Since the solvation of ethylene carbonate into lithium ions is facilitated by increasing the blending amount of ethylene carbonate in the non-aqueous solvent to 20% by volume or more, the effect of suppressing the decomposition of the solvent can be improved.
[0095]
A more preferable composition of the mixed solvent is EC / DMC, EC / PC / DMC, EC / DEC, EC / PC / DEC, EC / γ-BL / DEC mixed solvent, and the volume ratio of DMC or DEC is 60%. The following is preferable. Thus, by setting the ratio of DEC to 60% or less, more preferably 35% or less, the flash point of the mixed solvent can be increased, and safety can be improved. However, from the viewpoint of further reducing the viscosity, an ether such as diethoxyethane may be added in an amount of 30% by volume or less.
[0096]
Examples of main impurities present in the mixed solvent (non-aqueous solvent) include moisture and organic peroxides (for example, glycols, alcohols, carboxylic acids). Each of the impurities is considered to form an insulating film on the surface of the graphitized material and increase the interfacial resistance of the electrode. Therefore, the cycle life and capacity may be affected. In addition, self-discharge during storage at high temperatures (60 ° C. or higher) may increase. For this reason, it is preferable that the impurities be reduced as much as possible in the electrolyte solution containing the non-aqueous solvent. Specifically, the moisture is preferably 50 ppm or less and the organic peroxide is preferably 1000 ppm or less.
[0097]
Examples of the electrolyte contained in the non-aqueous electrolyte include lithium perchlorate (LiClO)._Four), Lithium hexafluorophosphate (LiPF)₆), Lithium borofluoride (LiBF)_Four), Lithium hexafluoroarsenide (LiAsF)₆), Lithium trifluorometasulfonate (LiCF)_ThreeSO_Three), Bistrifluoromethylsulfonylimide lithium [LiN (CF_ThreeSO₂)₂] Lithium salt (electrolyte). Among them, LiPE₆, LiBF_Four, LiN (CF_ThreeSO₂)₂Is preferably used. In particular, LiN (CF_ThreeSO₂)₂When there is used, there is little reaction with the positive electrode active material at high temperatures (for example, 60 ° C.), and excellent charge / discharge cycle characteristics can be obtained at high temperatures. Moreover, it has the advantage that it is stable with respect to the carbonaceous material and can improve the cycle life.
[0098]
The amount of the electrolyte dissolved in the non-aqueous solvent is preferably 0.5 to 2.0 mol / 1.
[0099]
【Example】
Hereinafter, an embodiment of the present invention will be described in detail with reference to FIG.
[0100]
Example 1
First, lithium cobalt oxide (Li_xCoO₂(0.8 ≦ x ≦ 1)) 91% by weight of powder, 3.5% by weight of acetylene black, 3.5% by weight of graphite and 2% by weight of ethylene propylene diene monomer powder and toluene were mixed together, and aluminum foil ( 30 [mu] m) After applying to a current collector, the positive electrode was produced by pressing.
[0101]
Moreover, after carbonizing coke made from low sulfur (content 8000 ppm or less) petroleum pitch as a raw material at 1000 ° C. in an argon atmosphere, an average particle size of 40 μm, a particle size of 1 to 80 μm, and 90% by volume are present, and A carbonaceous material was produced by appropriately pulverizing particles having a particle size of 0.5 μm or less so as to be small (5% or less) and then graphitizing at 3000 ° C. under vacuum.
[0102]
The obtained carbonaceous material is graphitized carbon powder having an average particle size of 40 μm, and 90% by volume or more exists in 1 to 80 μm in particle size distribution, and the particle size distribution of particles having a particle size of 0.5 μm or less is 0% by volume. Met. N₂Specific surface area by gas adsorption BET method is 3m²/ G. The shape of the powder was a block shape. Intensity ratio by X-ray diffraction (P₁₀₁/ P₁₀₀) Was 3.6. d₀₀₂Was 0.3354 nm, Lc was 43 nm, La was 43 nm, and La / Lc was 1. The ratio of rhombohedral and hexagonal (101) diffraction peak intensities by X-ray diffraction was 0.4. The sulfur content in the carbonaceous material was 100 ppm or less. In addition, the oxygen content was 100 ppm or less, the nitrogen content was 100 ppm or less, and Fe and Ni were each 1 ppm.
[0103]
Next, 96.7% by weight of the carbonaceous material was mixed with 2.2% by weight of styrene butadiene rubber and 1.1% by weight of carboxymethyl cellulose, and this was applied to a copper foil as a current collector and dried to form a negative electrode. Produced.
[0104]
The positive electrode, a separator made of a polyethylene porous film, and the negative electrode were laminated in this order, and then wound in a spiral shape so that the negative electrode was located on the outer side, thereby producing an electrode group.
[0105]
Furthermore, lithium hexafluorophosphate (LiPF₆) Was dissolved in a mixed solvent of ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) (mixed volume ratio 40:30:30) at 1.0 mol / 1 to prepare a non-aqueous electrolyte.
[0106]
The electrode group and the electrolyte solution were respectively housed in a bottomed cylindrical container made of stainless steel, and the above-described cylindrical lithium secondary battery shown in FIG. 1 was assembled.
[0107]
Example 2
First, after anisotropy bulk mesophase pitch obtained from petroleum pitch was carbonized at 1000 ° C. in an argon atmosphere, the average particle size was 15 μm, the particle size was 1 to 80 μm, and 90% by volume was present. The particles were moderately pulverized so that the number of particles having a diameter of 0.5 μm or less was small (5% or less). Then, the carbonaceous material was manufactured by heat-processing at 3000 degreeC under vacuum, and graphitizing.
[0108]
The obtained carbonaceous material is graphitized carbon powder having an average particle size of 15 μm, and 90% by volume or more exists in 1 to 80 μm in particle size distribution, and the distribution of particles having a particle size of 0.5 μm or less is 1% by volume. there were. N₂Specific surface area by gas adsorption BET method 7m²/ G. The shape of the powder was spherical. Intensity ratio by X-ray diffraction (P₁₀₁/ P₁₀₀) Was 2.6. d₀₀₂0.3357 nm, Lc was 45 nm, La was 58 nm, and La / Lc was 1.29. The ratio of rhombohedral and hexagonal (101) diffraction peak intensities was 0.5. Further, the sulfur content in the carbonaceous material was 100 ppm or less. In addition, the oxygen content was 100 ppm or less, the nitrogen content was 100 ppm or less, and Fe and Ni were each 1 ppm.
[0109]
A negative electrode was produced in the same manner as in Example 1 using the carbonaceous material. The cylindrical lithium secondary battery shown in FIG. 1 was assembled in the same manner as in Example 1 except that this negative electrode was used.
[0110]
Example 3
Mesophase pitch carbon fiber powder A carbonized at 900 ° C. has an average fiber length of 30 μm, an average fiber diameter of 7 μm, N₂Specific surface area by gas adsorption BET method 5m²/ G, Sulfur content 1600 ppm, Nitrogen content 200 ppm, Fe 5 ppm, Ni 3 ppm, X-ray diffraction, (002) plane spacing d of the graphite structure₀₀₂Showed a diffraction peak derived from 0.361 nm, Lc was 1.4 nm, and La was 3.9 nm.
[0111]
Mesophase pitch-based carbon fiber powder B graphitized at 3100 ° C. has an average fiber length of 40 μm, an average fiber diameter of 12 μm, N₂Specific surface area by gas adsorption BET method 3.8m²/ G, a sulfur content of 100 ppm or less, a nitrogen content of 100 ppm or less, and an interplanar spacing d of the (002) plane of the graphite structure in X-ray diffraction₀₀₂Shows a diffraction peak derived from 0.3365 nm, Lc is 37 nm, La is 67 nm, P₁₀₁/ P₁₀₀Was 2.3.
[0112]
A negative electrode was produced in the same manner as in Example 1 by using a carbonaceous material obtained by mixing the powder A and the powder B at a weight ratio of 1: 2. The cylindrical lithium secondary battery shown in FIG. 1 was assembled in the same manner as in Example 1 except that this negative electrode was used.
[0113]
Example 4
Mesophase pitch carbon fiber powder A carbonized at 1000 ° C. has an average fiber length of 30 μm, an average fiber diameter of 7 μm, N₂Specific surface area by gas adsorption BET method 5m²/ G, sulfur content 1500 ppm, nitrogen content 100 ppm, Fe 5 ppm, Ni 3 ppm, the spacing d of (002) plane of the graphite structure in X-ray diffraction₀₀₂Showed a diffraction peak derived from 0.360 nm, Lc was 1.48 nm, and La was 3.4 nm.
[0114]
Artificial graphite powder B obtained by graphitizing coke at 3000 ° C. has an average particle size of 40 μm, N₂Specific surface area by gas adsorption BET method 3.3m²/ G, the sulfur content is 100 ppm or less, the nitrogen content is 100 ppm or less, Fe and Ni are each 1 ppm, and the surface spacing d of the (002) plane of the graphite structure in X-ray diffraction₀₀₂Showed a diffraction peak derived from 0.3354 nm, Lc was 45 nm, and La was 60 nm.
[0115]
A negative electrode was produced in the same manner as in Example 1 using a carbonaceous material obtained by mixing the powder A and the powder B at a weight ratio of 1: 1. The cylindrical lithium secondary battery shown in FIG. 1 was assembled in the same manner as in Example 1 except that this negative electrode was used.
[0116]
Comparative Example 1
Natural graphite was pulverized to an average particle size of 6 μm to obtain graphite powder. In this graphite powder, 85% by volume of particles having a particle size distribution of 1 to 80 μm were present, and the distribution of particles having a particle size of 0.5 μm or less was 10% by volume. N₂Specific surface area by gas adsorption BET method is 10m²/ G. Intensity ratio by X-ray diffraction (P₁₀₁/ P₁₀₀) Was 2.1. d₀₀₂0.3357 nm, Lc was 33 nm, and La was 60 nm. La / Lc is 1.82. The ratio of rhombohedral and hexagonal (101) diffraction peak intensities was 0.85.
[0117]
A negative electrode was produced by the same method as in Example 1 using the graphite powder. The cylindrical lithium secondary battery shown in FIG. 1 was assembled in the same manner as in Example 1 except that this negative electrode was used.
[0118]
Comparative Example 2
A negative electrode was produced in the same manner as in Example 1 using only the artificial graphite powder B of Example 4. The cylindrical lithium secondary battery shown in FIG. 1 was assembled in the same manner as in Example 1 except that this negative electrode was used.
[0119]
Comparative Example 3
A negative electrode was produced in the same manner as in Example 1 using only the mesophase pitch-based carbon fiber powder A of Example 3. The cylindrical lithium secondary battery shown in FIG. 1 was assembled in the same manner as in Example 1 except that this negative electrode was used.
[0120]
The obtained lithium secondary batteries of Examples 1 to 4 and Comparative Examples 1 to 3 were charged and discharged at a charging current of 400 mA for 3 hours up to 4.2 V and discharged at a high rate current of 1 A up to 2.7 V. Repeatedly, the discharge capacity and cycle life of each battery were measured. The result is shown in FIG.
[0121]
As can be seen from FIG. 2, the lithium secondary batteries of Examples 1 to 4 have a higher capacity and a significantly improved cycle life even at high rate discharge than the batteries of Comparative Examples 1 to 3. I understand.
[0122]
On the other hand, LiN (CF_ThreeSO₂)₂When the same evaluation was performed using, it was confirmed that the cycle life was longer than those of Examples 1 to 4.
[0123]
Example 5
First, lithium cobalt oxide (Li_xCoO₂(0.8 ≦ x ≦ 1)) 91% by weight of powder, 3.5% by weight of acetylene black, 3.5% by weight of graphite and 2% by weight of ethylene propylene diene monomer powder and toluene were mixed together, and aluminum foil ( 30 [mu] m) After applying to a current collector, the positive electrode was produced by pressing.
[0124]
Moreover, after carbonizing coke made from low sulfur (content 8000 ppm or less) petroleum pitch as a raw material at 1000 ° C. in an argon atmosphere, an average particle size of 20 μm, a particle size of 1 to 80 μm, and 90% by volume are present, and A carbonaceous material was produced by pulverizing into blocks so that the number of particles having a particle size of 0.5 μm or less was reduced (5% or less) and then graphitizing at 3000 ° C. in an argon atmosphere.
[0125]
The obtained carbonaceous material is a block-shaped graphitized carbon powder having an average particle size of 20 μm, and 90% by volume or more exists in 1 to 80 μm in particle size distribution, and the particle size distribution of particles having a particle size of 0.5 μm or less is It was 0% by volume. N₂Specific surface area by gas adsorption BET method is 3m²/ G. The shape of the powder was granular. Intensity ratio by X-ray diffraction (P₁₀₁/ P₁₀₀) Was 3.6. d₀₀₂Was 0.3354 nm, Lc was 43 nm, La was 43 nm, and La / Lc was 1. The ratio of rhombohedral and hexagonal (101) diffraction peak intensities by X-ray diffraction was 0.4. The sulfur content in the carbonaceous material was 100 ppm or less. In addition, the oxygen content was 100 ppm or less, the nitrogen content was 100 ppm or less, and Fe and Ni were each 1 ppm.
[0126]
Next, 96.7% by weight of the carbonaceous material was mixed with 2.2% by weight of styrene butadiene rubber and 1.1% by weight of carboxymethyl cellulose, and this was applied to a copper foil as a current collector and dried to form a negative electrode. Produced.
[0127]
The positive electrode, a separator made of a polyethylene porous film, and the negative electrode were laminated in this order, and then wound in a spiral shape so that the negative electrode was located on the outer side, thereby producing an electrode group.
[0128]
Furthermore, lithium hexafluorophosphate (LiPF₆) Was dissolved in a mixed solvent of ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) (mixed volume ratio 40:30:30) at 1.0 mol / 1 to prepare a non-aqueous electrolyte.
[0129]
The electrode group and the electrolyte solution were respectively housed in a bottomed cylindrical container made of stainless steel, and the above-described cylindrical lithium secondary battery shown in FIG. 1 was assembled.
[0130]
Example 6
First, an anisotropic mesophase pitch obtained from petroleum pitch is spun and carbon fiber obtained by infusibilization is carbonized at 1000 ° C. in an argon atmosphere, and then an average fiber diameter of 15 μm and a particle size of 1 to 80 μm. Was appropriately pulverized so that 90% by volume was present and particles having a particle size of 0.5 μm or less were reduced (5% or less). Thereafter, a fibrous carbonaceous material was produced by graphitizing by heat treatment at a temperature of 3100 ° C. in an argon atmosphere. The carbonaceous material had a radial orientation in a cross section perpendicular to the fiber length direction.
[0131]
The obtained carbonaceous material is graphitized carbon powder having an average fiber diameter of 15 μm, and 90% by volume or more exists in 1 to 80 μm in particle size distribution, and the distribution of particles having a particle diameter of 0.5 μm or less is 1% by volume. there were. N₂Specific surface area 4m by gas adsorption BET method²/ G. The shape of the powder was a fiber having a length to diameter ratio (length / diameter) of 3. Intensity ratio by X-ray diffraction (P₁₀₁/ P₁₀₀) Was 2.6. d₀₀₂0.3357 nm, Lc was 45 nm, La was 58 nm, and La / Lc was 1.29. The ratio of rhombohedral and hexagonal (101) diffraction peak intensities was 0.5. Further, the sulfur content in the carbonaceous material was 100 ppm or less. In addition, the oxygen content was 100 ppm or less, the nitrogen content was 100 ppm or less, and Fe and Ni were each 1 ppm.
[0132]
A negative electrode was produced in the same manner as in Example 5 using the carbonaceous material. A cylindrical lithium secondary battery shown in FIG. 1 was assembled in the same manner as in Example 5 except that this negative electrode was used.
[0133]
Example 7
The mesophase pitch carbon fiber powder A graphitized at 3000 ° C. has an average fiber length of 40 μm, an average fiber diameter of 7 μm, N₂Specific surface area by gas adsorption BET method 3m²/ G, Sulfur content 1600 ppm, Nitrogen content 200 ppm, Fe 5 ppm, Ni 3 ppm, X-ray diffraction, (002) plane spacing d of the graphite structure₀₀₂Showed a diffraction peak derived from 0.3375 nm, Lc was 30 nm, and La was 60 nm.
[0134]
Petroleum coke powder B obtained by graphitization at 3100 ° C. and pulverization has an average particle size of 6 μm, N₂Specific surface area of 9m by gas adsorption BET method²/ G of flaky powder, having a sulfur content of 100 ppm or less and a nitrogen content of 100 ppm or less, and an interplanar spacing d of the (002) plane of the graphite structure in X-ray diffraction₀₀₂Shows a diffraction peak derived from 0.3358 nm, Lc is 60 nm, La is 120 nm, P₁₀₁/ P₁₀₀Was 2.5.
[0135]
A negative electrode was produced in the same manner as in Example 5 using a carbonaceous material obtained by mixing the powder A and the powder B at a weight ratio of 4: 1. A cylindrical lithium secondary battery shown in FIG. 1 was assembled in the same manner as in Example 5 except that this negative electrode was used.
[0136]
Example 8
The mesophase pitch microspheres graphitized at 3000 ° C. are obtained by oxidizing and removing a surface layer having an average particle diameter of 20 μm and a thickness of 1 to 5 μm from the surface to expose a lamellar layer. The mesophase pitch spherule is N₂Specific surface area by gas adsorption BET method is 5m²/ G, the sulfur content is 200 ppm, the nitrogen content is 100 ppm, Fe is 5 ppm, Ni is 3 ppm, and the distance d between the (002) planes of the graphite structure in X-ray diffraction₀₀₂Shows a diffraction peak derived from 0.3365 nm, Lc is 60 nm, La is 100 nm, intensity ratio (P₁₀₁/ P₁₀₀) Is a 2.3 spherical graphitized product.
[0137]
A negative electrode was produced in the same manner as in Example 5 using the graphitized product (carbonaceous material). A cylindrical lithium secondary battery shown in FIG. 1 was assembled in the same manner as in Example 5 except that this negative electrode was used.
[0138]
Comparative Example 4
Natural graphite was pulverized to an average particle size of 6 μm to obtain flaky graphite powder.
The graphite powder had a particle size distribution of 1 to 80 μm and 85% by volume, and the distribution of particles having a particle size of 0.5 μm or less was 5% by volume. N₂Specific surface area by gas adsorption BET method is 10m²/ G. Intensity ratio by X-ray diffraction (P₁₀₁/ P₁₀₀) Was 3.6. d₀₀₂0.3357 nm, Lc was 60 nm, and La was 120 nm. La / Lc is 1.82. The ratio of rhombohedral and hexagonal (101) diffraction peak intensities was 0.85.
[0139]
A negative electrode was produced in the same manner as in Example 5 using the graphite powder. A cylindrical lithium secondary battery shown in FIG. 1 was assembled in the same manner as in Example 5 except that this negative electrode was used.
[0140]
Comparative Example 5
A negative electrode was produced in the same manner as in Example 5 using only the graphitized petroleum coke powder B of Example 7. A cylindrical lithium secondary battery shown in FIG. 1 was assembled in the same manner as in Example 5 except that this negative electrode was used.
[0141]
Comparative Example 6
A negative electrode was prepared in the same manner as in Example 5 using only the mesophase pitch carbon fiber powder A of Example 7. A cylindrical lithium secondary battery shown in FIG. 1 was assembled in the same manner as in Example 5 except that this negative electrode was used.
[0142]
For the obtained lithium secondary batteries of Examples 5 to 8 and Comparative Examples 4 to 6, charging and discharging were performed at a charging current of 400 mA for 3 hours up to 4.2 V and discharged at a high rate current of 1 A up to 2.7 V. Repeatedly, the discharge capacity and cycle life of each battery were measured. The result is shown in FIG.
[0143]
As is clear from FIG. 3, the lithium secondary batteries of Examples 5 to 8 have a higher capacity and a significantly improved cycle life even at high rate discharge than the batteries of Comparative Examples 4 to 6. I understand.
[0144]
On the other hand, LiN (CF_ThreeSO₂)₂When the same evaluation was performed using, it was confirmed that the cycle life was longer than that of Examples 5 to 8.
[0145]
In addition, although the example applied to the cylindrical lithium secondary battery has been described in the above embodiment, the present invention can be similarly applied to a prismatic lithium secondary battery. The electrode group housed in the battery container is not limited to the spiral shape, and a plurality of positive electrodes, separators, and negative electrodes may be stacked in this order.
[0146]
【The invention's effect】
As described above in detail, according to the present invention, it is possible to provide a lithium secondary battery and a negative electrode material that have a high capacity, an excellent cycle life, and can maintain a high voltage over a long period of use.
[Brief description of the drawings]
FIG. 1 is a partial cross-sectional view showing a cylindrical lithium secondary battery according to the present invention.
FIG. 2 is a characteristic diagram showing a relationship between a charge / discharge cycle and a discharge capacity in the lithium secondary batteries of Examples 1 to 4 and Comparative Examples 1 to 3.
FIG. 3 is a characteristic diagram showing the relationship between charge / discharge cycles and discharge capacities in lithium secondary batteries of Examples 5 to 8 and Comparative Examples 4 to 6.
[Explanation of symbols]
1 ... container,
3 ... Electrode group,
4 ... positive electrode,
6 ... negative electrode,
8 ... Sealing plate.

Claims (7)

In a lithium secondary battery comprising a positive electrode, a negative electrode containing a carbonaceous material that occludes / releases lithium ions, and a non-aqueous electrolyte,
The carbonaceous material is located face spacing d ₀₀₂ of (002) plane of the graphite structure by X-ray diffraction method in the range of 0.335-0.33 6 nm, (101) of the hexagonal by X-ray diffraction diffraction The ratio of the rhombohedral (101) diffraction peak to the peak by the X-ray diffraction method is 0.6 or less , and the hexagonal (101) diffraction peak P ₁₀₁ and the (100) diffraction peak P by the X-ray diffraction method. A lithium secondary battery characterized in that a peak intensity ratio of ₁₀₀ (P ₁₀₁ / P ₁₀₀ ) exceeds 2.2 . The lithium secondary battery according to claim 1, wherein the carbonaceous material is a graphitized material mainly having a block shape, a fiber shape, or a spherical shape . The carbonaceous material, according to claim 1 or 2, wherein the ratio of the length Lc of the a-axis length in the direction of the crystallite La and the c-axis direction of the crystallite (La / Lc) is less than 1.5 The lithium secondary battery as described. The lithium secondary battery according to any one of claims 1 to 3, wherein the carbonaceous material has a sulfur content of 1000 ppm or less (including 0 ppm). The carbonaceous material has an oxygen content of 500 ppm or less, a nitrogen content of 1000 ppm or less, an Fe content of 50 ppm or less, and a Ni content of 50 ppm or less. The lithium secondary battery of any one of -4. The lithium secondary battery according to claim 1, wherein the carbonaceous material contains aluminum, boron, phosphorus, calcium, or tin. X-ray diffraction of the (002) plane spacing d ₀₀₂ of the graphite structure by X-ray diffraction method is in the range of 0.335 to 0.33 6 nm, and the hexagonal (101) diffraction peak by X-ray diffraction method. The ratio of the rhombohedral (101) diffraction peak obtained by the method is 0.6 or less , and the peak intensity ratio between the hexagonal (101) diffraction peak P ₁₀₁ and the (100) diffraction peak P ₁₀₀ obtained by the X-ray diffraction method A negative electrode material characterized by containing a carbonaceous material in which (P ₁₀₁ / P ₁₀₀ ) exceeds 2.2 .

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