JP5212682B2 - Method for producing negative electrode material for lithium ion secondary battery - Google Patents

Description

The present invention relates to a method for producing a negative electrode material for a lithium ion secondary battery that can be charged and discharged with a large current.

  A lithium secondary battery using a lithium salt organic electrolyte as a non-aqueous electrolyte secondary battery is lightweight and has high energy density, and is expected as a power source for small electronic devices or a battery for storing power. Initially, metallic lithium was used as the negative electrode material for lithium secondary batteries, but metallic lithium eluted into the electrolyte as lithium ions during discharge, and when lithium ions were deposited on the negative electrode surface as metallic lithium during charging. It is difficult to deposit in a smooth and original state, and it tends to deposit in a dendritic form. Since this dendrite has extremely strong activity, the electrolyte solution is decomposed, so that the battery performance is lowered and the charge / discharge cycle life is shortened. Furthermore, there is a risk that dendrites grow and reach the positive electrode, causing both electrodes to short-circuit.

  In order to remedy this drawback, it has been proposed to use a carbon material instead of metallic lithium. A carbon material, particularly a graphite material, is suitable as a negative electrode material because there is no problem of precipitation in the form of dendrites during insertion and extraction of lithium ions. In other words, the graphite material has high lithium ion occlusion / release properties, and since the occlusion / release reaction is performed quickly, the charge / discharge efficiency is high, the theoretical capacity is also high at 372 mAh / g, and the potential during charge / discharge is also high. There is an advantage that a high voltage battery is obtained which is almost equal to metallic lithium.

  However, in the case of a graphite material having a high degree of graphitization and a highly developed hexagonal network structure, the reversible capacity is large and high characteristics such as an initial efficiency of 90% or more can be obtained, but charging and discharging cannot be performed in a short time. The rate characteristic is bad.

  On the other hand, amorphous carbon having a low degree of graphitization and low crystallinity is characterized by excellent rate characteristics although it has a small reversible capacity.

  Therefore, by improving the properties of carbon materials centering on graphite materials, for example, a carbon material having a multi-layer structure in which the surface of a graphite material having a high graphitization degree is coated with a carbonaceous material having a low graphitization degree or a graphitization degree. Many attempts have been made to solve these problems by combining a high graphite material and a carbonaceous material having a low graphitization degree, and many proposals have been made.

For example, Patent Document 1 discloses a carbon negative electrode for a secondary battery in which a surface in contact with an electrolytic solution of carbon serving as an active material is covered with amorphous carbon, and amorphous carbon has a turbulent structure, and a C axis mean spacing directions 0.337～0.360Nm, the peak intensity ratio of 1360 cm ^-1 relative to 1580 cm ^-1 in an argon laser Raman spectrum secondary battery carbon negative electrode of 0.4 to 1.0 has been proposed .

In Patent Document 2, carbonaceous material (A) particles satisfying the following condition (1) and organic compound (B) particles satisfying the following condition (2) are mixed and heated (B ) Is carbonized, and an electrode material having a multilayer structure in which the particles of (A) are coated with a carbonaceous material (C) that satisfies the following condition (3) has been proposed.
(1) The d002 in X-ray wide angle diffraction is 3.37 angstroms or less, the true density is 2.10 g / cm ³ or more, and the volume average particle diameter is 5 μm or more.
(2) The volume average particle size is smaller than the carbonaceous material (A).
(3) d002 in X-ray wide angle diffraction 3.38 angstroms, in the Raman spectrum analysis using an argon ion laser beam, a peak in the range of 1580~1620cm ^-1 PA, a peak PB in the range of 1350 ^-1 The ratio R = IB / IA of the intensity IB of PB to the intensity IA of the PA is 0.2 or more.

  These form an amorphous carbon layer on the surface of the graphite particles, but it is difficult to form a thick amorphous carbon layer, so that graphitization proceeds easily and is not sufficient to improve the rate characteristics.

  In view of this, it has also been proposed to coat the surface of graphite particles with carbon black, which is one of amorphous carbon, and Patent Document 3 discloses particle characteristics having an absorption average of 100 ml / 100 g or more and an arithmetic average primary particle diameter of 40 nm or more. There is disclosed a lithium secondary battery in which lithium as a negative electrode active material is supported on carbon black having a negative electrode body, and ethylene carbonate or an organic solvent containing 20% by volume or more thereof is used as an electrolyte.

Patent Document 4 discloses a negative electrode body of a lithium secondary battery comprising a carbon material capable of occluding lithium as a negative electrode active material and a binder, and the carbon material is carbon black having a DBP absorption of 100 ml / 100 g or more. A negative electrode body for a lithium secondary battery is disclosed, wherein the binder is polyvinylidene fluoride.
JP-A-4-368778 JP-A-6-267531 JP-A-6-267533 JP-A-7-153447

  However, since carbon black has a large specific surface area and carbon black particles form an aggregate (structure), loss (charge capacity-discharge capacity) increases, and the carbon black layer covering the graphite material increases. There is a disadvantage that the density is also lowered.

  Therefore, the inventors conducted extensive research on the development of a negative electrode material that combines these characteristics, taking advantage of the characteristics such as the large reversible capacity and high initial efficiency of the graphite material and the excellent rate characteristics of the amorphous carbon material. . Then, a core-shell structure composite particle in which graphite powder having developed graphite crystal properties is used as a core, and the surface of the graphite powder is coated with an amorphous carbon layer containing amorphous carbon powder. It was confirmed that battery performance such as high reversible capacity, initial efficiency, and rate characteristics can be imparted by specifying the particle size of the composite particles.

That is, the objective of this invention is providing the manufacturing method of the negative electrode material for lithium ion secondary batteries which has the outstanding characteristic.

In order to achieve the above object, a method for producing a negative electrode material for a lithium ion secondary battery according to the present invention comprises a graphite powder having an average particle diameter D50 of 10 to 30 μm and an average lattice spacing d (002) of 0.336 nm or less. After mixing 100 parts by weight and 10 to 50 parts by weight of a binder pitch having a quinoline insoluble content of 3% by weight or more and kneading, the average particle diameter D50 is 0.2 to 5 μm and the average lattice spacing d (002). Is characterized by adding 10 to 50 parts by weight of amorphous carbon powder of 0.340 nm or more and further kneading, followed by heat treatment at a temperature of 800 to 2200 ° C. in a non-oxidizing atmosphere.

  As the amorphous carbon powder, mosaic coke powder is suitable.

ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the negative electrode material for lithium ion secondary batteries provided with the high reversible capacity | capacitance, initial stage efficiency, and a rate characteristic with sufficient balance is provided.

  The reversible capacity is the amount of electricity that can be reversibly charged and discharged. The initial efficiency is a dischargeable capacity with respect to the capacity charged for the first time in constant current charging / discharging, and is defined by the initial efficiency (%) = (initial discharge capacity) / (initial charge capacity) × 100. It is. The rate characteristic is an index indicating whether or not it can withstand a discharge with a large current, and is represented by a value obtained by dividing the amount of electricity when discharged with a large current by the amount of electricity when discharged with a small current. If the rate characteristic is low, it cannot be used for applications requiring a large current.

  In lithium ion secondary batteries, lithium ions enter the graphite layer of the negative electrode material during charging to produce an intercalation compound, and during discharge, lithium ions are desorbed from the graphite layer and return to the positive electrode. Depends on the speed of entry and desorption of lithium ions. When the speed at the time of entry is compared with the speed at the time of desorption, the speed at the time of desorption is sufficiently faster than the speed at the time of entry. On the other hand, comparing the speed at which lithium ions enter the negative electrode material from the electrolyte interface and the speed at which the lithium ions enter and move inside the negative electrode material, the speed at which the lithium ions enter and move inside the negative electrode material is faster.

  That is, the most rate-determining step governing the rate characteristics is the speed at which lithium ions enter the negative electrode material from the electrolyte solution interface. This speed depends on the degree of graphite crystallinity of the negative electrode material. The highly graphitized negative electrode material has a slow entry speed, and amorphous carbon having a low degree of crystallinity has a relatively high speed. On the other hand, the reversible capacity is simply proportional to the degree of graphitization of the negative electrode material.

The negative electrode material for a lithium ion secondary battery manufactured according to the present invention has amorphous carbon where the lithium ions that govern rate characteristics enter the negative electrode material from the interface with the electrolyte, and the lithium ions that have entered diffused and moved into the interior. Then, the part that generates the intercalation compound is a graphite material with a large reversible capacity, and the graphite powder with developed graphite crystal properties is used as a core, and the surface of the graphite powder is coated with an amorphous carbon layer containing amorphous carbon powder. It is a composite particle having a core / shell structure.

  In this composite particle, the average particle diameter D50 of the graphite powder is 10-30 μm. If it is smaller than 10 μm, the loss increases due to the increase in the specific surface area, and if it exceeds 30 μm, the diffusion path of lithium ions inside the graphite. This is because the rate characteristic is lowered.

  The average particle diameter D50 of the amorphous carbon powder that forms the shell by covering the surface with this graphite powder as the core is 0.2 to 5 μm. On the other hand, if it exceeds 5 μm, it is difficult to form a shell and it is impossible to coat uniformly, resulting in a deterioration in rate characteristics.

  The composite particles are obtained by binding the graphite powder and the amorphous carbon powder with a carbide of binder pitch, and covering the peripheral surface with an amorphous carbon layer containing the amorphous carbon powder with the graphite powder as a core. As a result of using the graphite powder and the amorphous carbon powder, the average particle diameter D50 of the composite particles is 11 to 40 μm.

  In addition, if the surface of the graphite powder is coated with an amorphous carbon layer containing amorphous carbon powder, 60 to 100%, sufficient rate characteristics can be obtained, but if the coating ratio is less than 60%, the amorphous carbon layer is interposed. The rate of lithium ions that enter and the rate characteristics decrease. The covering ratio is calculated by selecting 100 or more particles from the surface observation photograph of the negative electrode material observed with a scanning electron microscope, and geometrically measuring the area of each uncoated portion.

  The reason why the thickness of the amorphous carbon layer is 0.5 to 5 μm is that sufficient rate characteristics cannot be obtained when the thickness is 0.5 μm or less, and the amount of amorphous carbon having a small reversible capacity is excessive when the thickness is 5 μm or more. It is because it falls. The thickness of the amorphous carbon layer is a value obtained by cutting the composite particles with an appropriate cutting device such as a focused ion beam device, measuring the cut surface with a scanning electron microscope, and measuring 100 visual fields.

  By using this composite particle to form a negative electrode material for a lithium ion secondary battery, a lithium ion secondary battery having a good balance between the excellent rate characteristics of amorphous carbon, the high reversible capacity of graphite and the initial efficiency is provided. be able to.

  The negative electrode material for a lithium ion secondary battery comprising the composite particles has an average particle diameter D50 of 10 to 30 μm and an average lattice spacing d (002) of 100 parts by weight of graphite powder of 0.336 nm or less and a binder pitch of 10 to 50 weights. After mixing and kneading the parts, 10 to 50 parts by weight of amorphous carbon powder having an average particle diameter D50 of 0.2 to 5 μm and an average lattice spacing d (002) of 0.340 nm or more is added and further kneaded, It is manufactured by heat treatment at a temperature of 800 to 2200 ° C. in a non-oxidizing atmosphere.

  As graphite, artificial graphite or natural graphite is used, and 100 parts by weight of graphite powder having an average particle diameter D50 of 10 to 30 μm and an average lattice spacing d (002) of 0.336 nm or less and a binder pitch of 10 to 50 parts by weight are used. Mix and knead. The graphite powder is pulverized by a pulverizer such as a vibration ball mill, a jet pulverizer, a roller mill, or a collision pulverizer, and then classified to adjust the particle size to a predetermined particle size.

  The graphite powder having an average particle diameter D50 of 10 to 30 μm is used when the D50 is smaller than 10 μm, because the specific surface area is increased and the loss increases. When the D50 is larger than 30 μm, the diffusion path of lithium ions inside the graphite becomes longer. This is because the rate characteristic is lowered, and the graphite powder having an average lattice spacing d (002) of 0.336 nm or less is used when the graphite powder is larger than 0.336 nm. This is because a lithium ion secondary battery having the same cannot be obtained.

  When the mixing ratio of the binder pitch is less than 10 parts by weight, the pitch amount is small and the graphite surface cannot be uniformly wetted. On the other hand, if it exceeds 50 parts by weight, the amount of pitch becomes excessive, and a kneaded granulated product of only amorphous carbon powder added later is formed.

  Binder pitch to be used includes those obtained by distillation, thermal polycondensation, extraction, chemical polycondensation of tars obtained by high-temperature pyrolysis such as coal tar, ethylene bottom oil, crude oil, etc., or wood dry distillation An example of the pitch that is sometimes generated is illustrated, and it may be liquid when kneaded. Moreover, when the content of the quinoline insoluble component in the binder pitch is 3% by weight or more, graphitization is difficult to proceed and the rate characteristics are improved, which is more preferable.

  The graphite powder and the binder pitch are mixed at a desired weight ratio and well heated and kneaded. As the kneader, a commercially available appropriate kneader can be used.

  Next, an amorphous carbon powder having an average particle diameter D50 of 0.2 to 5 [mu] m and an average lattice spacing d (002) of 0.340 nm or more is added to the kneaded material in a range of 10 to 50 parts by weight and further kneaded. Examples of the amorphous carbon powder include pulverized products of carbon black, coke, or resin carbide, but a mosaic-shaped coke pulverized product is preferably used.

  The average particle diameter D50 of the amorphous carbon powder to be used is set to 0.2 to 5 [mu] m. If D50 is smaller than 0.2 [mu] m, the specific surface area increases and the loss increases. On the other hand, if it exceeds 5 [mu] m, the shell formation is difficult and uniform. This is because the rate characteristics are degraded as a result. The average lattice spacing d (004) of 0.340 nm or more is used because the crystallinity of graphite is high when the average lattice spacing d (004) is smaller than 0.340 nm. This is because of the decrease.

  The amount of the amorphous carbon powder added is 10 to 50 parts by weight with respect to 100 parts by weight of the graphite powder because the surface of the graphite constituting the core part of the composite particle is less than 10 parts by weight with respect to 100 parts by weight of the graphite powder. This is because 60% or more cannot be covered, and the thickness of the amorphous carbon layer is less than 0.5 μm, so that the rate of lithium ions entering through the amorphous carbon layer is reduced and the rate characteristics are lowered. However, if the addition amount exceeds 50 parts by weight, there will be excessive amorphous carbon with originally low reversible capacity, and the thickness of the amorphous carbon layer will also exceed 5 μm, and the reversible capacity will be significantly reduced.

  The kneaded product is a composite in which amorphous carbon is coated and bound on the surface of graphite by heat-treating the binder pitch by carbonization by heating at a temperature of 800 to 2200 ° C. in a non-oxidizing atmosphere such as inert gas or nitrogen gas. A negative electrode material for a lithium ion secondary battery of the present invention comprising particles is produced.

  The average particle diameter D50 and the average lattice spacing d (002) in the present invention are measured values by the following method.

Average particle size D50;
The particle size distribution was measured with a laser diffraction particle size distribution analyzer (SALD2000 manufactured by Shimadzu Corporation), and the median diameter (μm) based on volume was shown.

Average lattice spacing d (002);
Using CuKα rays monochromatized with a graphite monochromator, a wide-angle X-ray diffraction curve is measured by the reflective diffractometer method, and measured using the Gakushin method.

  Examples of the present invention will be specifically described below in comparison with comparative examples. However, the present invention is not limited by these examples.

Example 1
To 100 parts by weight of artificial graphite powder having an average particle diameter D50 of 15 μm and an average lattice spacing d (002) of 0.3359 nm, 30 parts by weight of a softening temperature of 90 ° C. and a pitch of quinoline insoluble (QI) 5.6% are added. And kneaded. Thereafter, 50 parts by weight of mosaic coke A (manufactured by Nippon Steel Chemical Co., Ltd., LPC) having an average particle diameter D50 of 0.5 μm and an average lattice spacing d (002) of 0.3492 μm was added and further kneaded.

  After cooling, the kneaded product was put in a heating furnace and heat-treated in a nitrogen gas atmosphere at a temperature of 2000 ° C. to carbonize pitch and coke to produce composite particles. The average particle diameter D50 of the composite particles was 17 μm, and from the result of SEM observation, 85% of the surface of the artificial graphite powder was covered with the amorphous carbon layer.

Examples 2 to 13 and Comparative Examples 1 to 15
Based on the production conditions of Example 1, 100 parts by weight of graphite powder having different average particle diameter D50 and average lattice spacing d (002) were mixed and kneaded with different parts by pitch. Subsequently, mosaic coke (coke A) and needle coke (coke B) having different average particle diameter D50 and average lattice spacing d (002) are added in different parts by weight and further kneaded, and then the kneaded product is subjected to different temperatures. To produce composite particles.

  In Comparative Example 14, pitch and coke were simultaneously added to and kneaded with artificial graphite powder based on the production conditions of Example 1, and then the kneaded product was heat-treated. In Comparative Example 15, pitch was added to coke. After artificial graphite powder was added and kneaded, the kneaded product was heat-treated.

In Example 13, vapor phase grown carbon spheres having an average particle diameter D50 of 0.2 μm (specific surface area 8 m ² / g) and an average lattice spacing d (002) of 0.3582 nm were obtained by changing to coke as amorphous carbon powder. Comparative Example 13 is a carbon black having an average particle diameter D50 of 0.062 μm and an average lattice spacing d (002) of 0.3594 nm (Tokai Carbon Co., Seast SVH). It was used. In addition, the average particle diameter of carbon black used the value which measured the basic particle diameter of each carbon black basic particle diameter which comprises a carbon black particle aggregate by the electron microscope, and was arithmetically averaged.

  Table 1 shows the characteristics of the raw material system for producing these composite particles, the mixing parts by weight, the heat treatment temperature, and the like.

  Next, the average particle diameter D50 of these composite particles was measured, and the surface coverage by amorphous carbon on the surface of the graphite powder was measured by SEM observation.

  A lithium battery was assembled using the composite particles produced in this manner as a negative electrode material, battery characteristics were evaluated by the following method, and the results are also shown in Table 2.

Reversible capacity and initial efficiency;
After preparing a tripolar test cell with metallic lithium as the negative electrode and reference electrode and each graphite powder as the positive electrode, charging the lithium reference electrode with a constant current up to 0.002 V (inserting lithium ions into the graphite) The battery was discharged at a constant current up to 1.2 V (lithium ions were desorbed from the graphite), and the ratio of the amount of discharged electricity to the initial amount of charged electricity was defined as the initial efficiency. Furthermore, charge / discharge was repeated under the same conditions, and the reversible capacity per gram of graphite was calculated from the amount of electricity that was discharged (lithium ions were desorbed from the graphite) in the fifth cycle.

Rate characteristics (rapid discharge efficiency);
A coin-type battery was manufactured and discharged at a constant current from a fully charged state for 5 hours (0.2 C discharge current value), assuming a discharge capacity of 100% and 30 minutes (2.0 C discharge current value). ) And the discharge capacity when completely discharged was calculated as the ratio. Note that the discharge capacity decreases as the discharge current value increases and as the time for reaching the complete discharge state decreases.

  When the blending ratio of the amorphous carbon powder is too small, the coverage of the composite particles is small and the rate characteristics are lowered, and when it is excessive, the reversible capacity is lowered because the coating thickness is too thick (Examples 1 to 3, Comparative Example). 1-2). In addition, when the particle size of the amorphous carbon or the particle size of the graphite powder is out of the claimed range, the initial efficiency or rate characteristics deteriorate (Examples 4 to 7, Comparative Examples 3 to 6). When the binder pitch amount is out of the claims, the coverage is small and the rate is lowered (Examples 8 to 9, Comparative Examples 7 to 8). If the heat treatment temperature is low, the carbonization of amorphous carbon such as pitch and coke is insufficient, so that the reversible capacity is remarkably reduced. If it is too high, the graphitization of pitch carbide and amorphous carbon proceeds and the rate decreases (Examples 10-10). 11, Comparative Examples 9 to 10).

  The graphite powder having an average lattice spacing d (002) of 0.336 nm or more and the amorphous carbon powder having an average lattice spacing d (002) of 0.340 nm or less can be regarded as the graphite powder and the amorphous carbon powder described in claim 1. Therefore, the reversible capacity or rate characteristics are inferior (Example 12, Comparative Examples 11 to 12). Vapor growth carbon (carbon black) having a small particle size has a remarkably low initial efficiency (Example 13, Comparative Example 13). If the manufacturing procedure defined in claim 2 is not followed, the composite particles of claim 1 cannot be obtained and sufficient rate characteristics cannot be obtained (Comparative Examples 14 and 15).

Claims (1)

100 parts by weight of graphite powder having an average particle diameter D50 of 10 to 30 μm and an average lattice spacing d (002) of 0.336 nm or less and 10 to 50 parts by weight of binder pitch having a quinoline insoluble content of 3% by weight or more are mixed. And kneading, adding 10 to 50 parts by weight of amorphous carbon powder having an average particle diameter D50 of 0.2 to 5 μm and an average lattice spacing d (002) of 0.340 nm or more, and then non-oxidizing A method for producing a negative electrode material for a lithium ion secondary battery, wherein the heat treatment is performed at a temperature of 800 to 2200 ° C in a neutral atmosphere.