JP7185650B2 - Coating pitch for lithium ion secondary battery negative electrode material and method for producing the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to a coating pitch for lithium ion secondary battery negative electrode material and a method for producing the same.

Lithium-ion secondary batteries are widely used in mobile electronic devices, and are beginning to be used in hybrid vehicles and electric vehicles. Under these circumstances, lithium-ion secondary batteries are required to have even higher capacity, high-speed charging/discharging characteristics, and cycle characteristics.

Pitch with a high softening point used for coating negative electrode materials for lithium ion secondary batteries is usually produced by thermally modifying soft pitch.

Soft pitch is a residue obtained by blending tars generated from various coke ovens and distilling low-boiling oils such as naphthalene oil from the blends. This soft pitch has a softening point as low as 30 to 60 ° C., and the light content is excessive and the heavy content is insufficient. to manufacture. This reduced-pressure pitch is thermally reformed by the method described in Patent Document 1 to concentrate the softening point to 150 to 350°C. Specifically, heat treatment is performed at 420° C. or lower, preferably 395 to 400° C., while blowing a non-oxidizing gas such as nitrogen gas. By blowing in this gas, the temperature of the molten pitch is kept uniform, the uneven distribution of the pitch components is prevented, and at the same time, low boiling point substances present in the pitch are forcibly removed.

A high softening point and a low ash content are among the qualities required for coating pitch for lithium ion secondary battery negative electrode materials. The reason why the low ash content is required is that the metal content in the pitch adversely affects the electrode properties. However, although the softening point can be raised by conventional production methods, it has been difficult to lower the tar-derived ash content.

JP-A-10-139410

Accordingly, an object of the present invention is to provide a coating pitch for a lithium ion secondary battery negative electrode material having a high softening point and a low ash content, and a method for producing the same.

That is, the present invention provides the following coating pitch for lithium ion secondary battery negative electrode material and a method for producing the same.
[1] a softening point of 150 to 400° C.,
0.3 to 75% by mass of quinoline insolubles,
A pitch for coating a lithium ion secondary battery negative electrode material having an ash content of less than 0.1% by mass and a fixed carbon content of 60% by mass or more.
[2] A sedimentation step of separating into a light liquid and a heavy liquid by sedimentation after mixing coal tar pitch and an aromatic solvent;
a distillation step of distilling the light liquid to obtain soft pitch;
a vacuum distillation step of further vacuum-distilling the soft pitch to obtain vacuum pitch;
For coating a lithium ion secondary battery negative electrode material according to [1], comprising a heat treatment step of obtaining the pitch according to 1 by heat treatment at 395 to 420 ° C. while blowing a non-oxidizing gas into the reduced pressure pitch. How the pitch is made.

According to the present invention, a coating pitch for lithium ion secondary battery negative electrode material having a high softening point and a low ash content can be obtained.

FIG. 1 is a graph showing the relationship between heat treatment time and softening point for coating pitches of Examples 1-5.

The present invention will be described in more detail below.
The coating pitch for the lithium ion secondary battery negative electrode material of the present invention (hereinafter referred to as "coating pitch" in this specification) has a softening point of 150 to 400 ° C. and a quinoline insoluble content of 0.3 to 75% by weight, less than 0.1% by weight ash, and 60% by weight or more fixed carbon.

<Softening point (SP)>
The softening point (SP) of the coating pitch of the present invention is 150-400°C, preferably 230-370°C.
The softening point (SP) of the pitch for coating is a temperature index at which the fluidity of the pitch for coating is exhibited. This index is particularly important in the step of forming a kneaded product of coating pitch and negative electrode material. If the softening point of the coating pitch is within this range, uniform coating can be achieved.
The method for measuring the softening point (SP) is preferably 8.1 (manual measurement method) or 8.2 (automatic measurement method) of JIS K 2425: 2006 (creosote oil, processed tar and tar pitch test method). , 8.1 (manual measurement method) are more preferred.

<Quinoline insoluble (QI) content>
Quinoline insoluble matter (QI) is a component of heavy matter, and free carbon (particle diameter of about 1 μm or less), which is a gas phase generated carbon generated when coal is carbonized, and generated when pitch is thermally reformed The former is called primary QI, and the latter is called secondary QI.
The quinoline insoluble (QI) content of the coating pitch of the present invention is 0.7 to 75% by mass, preferably 30 to 75% by mass.
When the quinoline insoluble matter (QI) content of the pitch for coating is within this range, fusion between the negative electrode materials can be prevented during firing of the carbon material, and the amount of fixed carbon (FC) (carbonization yield) decreases. It is possible to avoid adverse effects such as falling.
The method for measuring the amount of quinoline insolubles (QI) is preferably 15.1 (filtration method) or 15.2 (centrifugation method) of JIS K 2425: 2006 (test methods for creosote oil, processed tar and tar pitch). .1 (filtration method) is more preferred.

<Ash content>
Ash is mainly composed of metal components such as iron and aluminum.
The amount of ash in the coating pitch of the present invention is 0.1% by mass, preferably 0.05% by mass or less.
When the ash content of the coating pitch is within this range, it is possible to prevent deterioration of the cycle characteristics of the lithium ion secondary battery.
The ash content is measured by heating a sample in air in a muffle furnace or the like at 550°C for 6 hours and determining the residue as ash.

<Fixed carbon (FC)>
In the coating pitch of the present invention, the amount of fixed carbon (FC) is 60% by mass or more, preferably 80% by mass or more. The amount of fixed carbon (FC) is an indicator of carbonization yield.
The method for measuring the amount of fixed carbon (FC) is preferably JIS K 2425:2006 (testing methods for creosote oil, processed tar and tar pitch), 11 (fixed carbon content determination method).

<Toluene insoluble (TI) content>
The coating pitch of the present invention preferably has a toluene insoluble (TI) content of 55 to 90% by mass, more preferably 60 to 85% by mass.
The method for measuring the toluene insolubles (TI) content is preferably JIS K 2425: 2006 (Methods for testing creosote oil, processed tar and tar pitch), 14.2 (Method for determining toluene insolubles in processed tar and tar pitch). .

[Method for producing coating pitch]
The method for producing pitch for coating of the present invention includes a precipitation step of mixing coal tar pitch and an aromatic solvent, followed by precipitation to separate the light liquid and the heavy liquid, and distilling the light liquid to obtain soft pitch. A distillation step, a vacuum distillation step of further distilling the soft pitch under reduced pressure to obtain a reduced pressure pitch, and a heat treatment at 395 to 420° C. while blowing a non-oxidizing gas into the reduced pressure pitch to obtain the coating pitch of the present invention. and a step.

(1) Sedimentation process (coal tar pitch)
Coal tar pitch is a residue obtained by distilling coal tar generated during coke production to distill off low-boiling oil such as naphthalene oil. It may also simply be called a pitch.
In this step, a mixture of multiple coal tar pitches may be used.

(Aromatic solvent)
As an aromatic solvent,
1) Aromatic solvents such as benzene and toluene having a boiling point of 180°C or less, or mixed solvents thereof,
2) Light oil containing benzene and toluene as main components,
3) gas light oil mainly composed of components with a boiling point of 250 ° C or less,
is preferred.

The dissolving power of these aromatic solvents for coal tar pitch is relatively small. For example, when dissolved in a sufficient amount of benzene, high molecular weight components such as quinoline insolubles (QI) become insoluble components. Toluene is almost the same as benzene and has a small dissolving power.
Therefore, if coal tar pitch and aromatic solvent are mixed and the solvent ratio (mixing ratio of aromatic solvent to coulter pitch) and the temperature and pressure in the separation zone are appropriately maintained, the insoluble components can be rapidly removed. Gravitational sedimentation can be used to separate a light liquid containing components with a small molecular weight and a heavy liquid containing components with a large molecular weight. Ash can be separated as a heavy liquid because it mainly consists of metal components such as iron and aluminum.

The specific gravity and viscosity of the aromatic solvent decrease with increasing temperature. In this state, the sedimentation speed of the undissolved components is much faster than at room temperature. In order to keep the aromatic solvent below the critical temperature, it is necessary to apply a predetermined pressure to keep the aromatic solvent in the liquid phase.
By appropriately maintaining the temperature and pressure in the separation zone, the light liquid and heavy liquid can be separated very efficiently. The temperature and pressure in the separation zone are preferably maintained at 150° C. or higher and 5 atm or higher, more preferably 150 to 350° C. and 15 to 60 atm.

(2) Distillation step In this step, the light liquid separated in the sedimentation step is distilled under normal pressure to remove the light components present in the light liquid, thereby obtaining soft pitch as a fraction having a boiling point of 280°C or higher.
Soft pitch is not generally defined by the type of light liquid used, but preferably 100 parts by weight of the light liquid charged is 75 parts by weight or less, preferably 70 parts by weight or less. can get.

(3) Vacuum Distillation Step In this step, the soft pitch obtained in the distillation step is further distilled under reduced pressure to obtain vacuum pitch. Vacuum distillation is carried out by further distilling the above atmospheric distillate under reduced pressure to remove residual light components in the atmospheric distillate. The atmospheric distillate is heated in a heating furnace, and the heating temperature is 360° C. or less, preferably 300 to 360° C., and vacuum distillation is performed in a short time under a reduced pressure of 50 Torr or less, preferably 30 Torr or less. Most of the light components are removed by this vacuum distillation, and pitches having various softening points are obtained depending on the vacuum distillation conditions. Conditions such as heating temperature, heating time, degree of pressure reduction, etc. are set so that the temperature is in the range of ~115°C, and distillation is carried out under reduced pressure.

(4) Heat treatment step In this step, the reduced pressure pitch obtained in the reduced pressure distillation step is heat treated at 395 to 420°C while blowing a non-oxidizing gas such as nitrogen gas. Thereby, mesophasing is promoted to obtain the coating pitch of the present invention.
By blowing in the non-oxidizing gas, the temperature of the molten pitch is kept uniform, the uneven distribution of pitch components is prevented, and low-boiling-point substances present in the pitch are forcibly removed. When the flow rate of the non-oxidizing gas is 0.05 Nm ³ /pitch kg·hr or more, preferably 0.15 to 0.20 Nm ³ /pitch kg·hr, the heat treatment time is usually about 3 to 7 hours. , preferably about 4-6 hours.

EXAMPLES Next, the present invention will be specifically described with reference to Examples, but the present invention is not limited to these Examples.

(Examples 1 to 5)
A coating pitch was obtained by the following procedure.

(Sedimentation process)
Coal tar pitch is used as a raw material, benzene is added as a solvent, and the mixture is thoroughly mixed. The mixture is led to a separation zone where the temperature is maintained at 280° C., the pressure is 45 atm, and the residence time is 2 minutes for gravitational sedimentation to form a light liquid and a heavy liquid. separated. The pitch used as a raw material and the composition of the light liquid after separation are shown below.
coal tar pitch
Quinoline insoluble component: 2.86% by mass
Quinoline soluble pyridine insoluble component: 3.89% by mass
Pyridine-soluble benzene-insoluble component: 13.05% by mass
light liquid
Softening point: 83.1°C
QI: 0% by mass
TI: 16% by mass
Fixed carbon (FC): 55% by mass
Ash content (Ash): 0.01% by mass

(Distillation process)
1,000 kg of the light liquid obtained in the sedimentation process was put into a reactor and distilled under normal pressure to obtain soft pitch.
(Reduced pressure distillation process)
The soft pitch obtained in the distillation step was subjected to reduced pressure distillation (340° C., 20 Torr) to obtain reduced pressure pitch.
(Heat treatment process)
The vacuum pitch obtained in the vacuum distillation step was heat-treated at the temperature and for the time shown in the table below while blowing nitrogen at the amount shown in the table below to obtain pitch for coating.
Table 1 shows the results.

(Comparative example 1)
Using the pitch of the above composition as a raw material, without performing the sedimentation process, 1,000 kg of the pitch is atmospherically distilled in the same procedure as described above to obtain soft pitch. After vacuum distillation is carried out according to the procedure to obtain a vacuum pitch, the vacuum pitch obtained in the vacuum distillation step is heat-treated at the temperature and for the time shown in the table below while blowing nitrogen at the blowing amount shown in the table below. got a pitch for
Table 1 shows the results.

The coating pitches obtained in Examples 1 to 5 have a softening point of 150 to 400 ° C., a quinoline insoluble content (QI) of 0.3 to 75% by mass, a fixed carbon (FC) of 60% by mass or more, and an ash content ( Ash) was less than 0.1% by mass.
On the other hand, the coating pitch obtained in Comparative Example 1 had a softening point of 150 to 400° C., a quinoline insoluble content (QI) of 0.3 to 75% by mass, and a fixed carbon (FC) of 60% by mass or more. However, the ash content (Ash) was more than 0.1% by mass.

FIG. 1 shows the relationship between the heat treatment time and the softening point for the coating pitches of Examples 1 to 5. The softening point sharply rises when the heat treatment time is around 250° C. This is considered to be because the heat treatment accelerates the mesophasing of the pitch.
Natural graphite was coated with the coating pitch produced in the example to fabricate a lithium ion secondary battery. When the battery characteristics (capacity, cycle characteristics, etc.) of the prototype battery were evaluated, good results were obtained.

Claims (2)

a softening point of 150-400°C;
0.3 to 75% by mass of quinoline insolubles,
A pitch for coating a lithium ion secondary battery negative electrode material having an ash content of less than 0.1% by mass and a fixed carbon content of 60% by mass or more. a sedimentation step of separating a light liquid and a heavy liquid by sedimentation after mixing coal tar pitch and an aromatic solvent;
a distillation step of distilling the light liquid to obtain soft pitch;
a vacuum distillation step of further vacuum-distilling the soft pitch to obtain vacuum pitch;
2. For coating a lithium ion secondary battery negative electrode material according to claim 1, comprising a heat treatment step of obtaining the pitch according to 1 by heat treatment at 395 to 420 ° C. while blowing a non-oxidizing gas into the reduced pressure pitch. How the pitch is made.

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