JP2844303B2 - Lithium secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery using a carbon anode material suitable for a lithium secondary battery.

[0002]

2. Description of the Related Art Lithium primary batteries using an organic solvent as an electrolyte and lithium metal as a negative electrode are widely used because of their many advantages. In the negative electrode of this battery, during use (during discharge), metallic lithium is oxidized and eluted into lithium ions, and simultaneously generated free electrons are supplied to an external circuit.

[0003] If an attempt is made to use this lithium primary battery as it is as a chargeable / dischargeable secondary battery by taking advantage of its excellent performance, it is supplied from an external circuit at the time of charging, contrary to the time of discharging at the above-mentioned metal lithium electrode. Lithium ions are reduced by the free electrons, and metallic lithium is deposited.

[0004]

However, when metallic lithium is deposited on the negative electrode, the metallic lithium precipitates in the form of particles or dendrites, so that the negative electrode does not return to its original state, and a short circuit between the positive and negative electrodes, etc. Since it causes various obstacles, it cannot be used as a secondary battery that repeatedly charges and discharges reversibly.

In a recent study, when a carbon material was used for the negative electrode instead of metallic lithium, the carbon material reversibly took in (doped) and released (undoped) lithium ions,
It has been proposed that it can be used as a chargeable / dischargeable secondary battery, and is being put to practical use. That is, the carbon material of the negative electrode is charged by doping lithium ions, and the lithium ions are undoped during discharging. Therefore, the charge capacity at the negative electrode is determined by the doping amount of lithium ions in the carbon material, and the discharge capacity at the negative electrode is determined by the undoped amount.

As a carbon material used for the negative electrode of the lithium secondary battery, graphite, which can take in lithium ions as an interlayer compound, has been noticed at first. in this case,
The ratio of one lithium atom to six carbon atoms is theoretically the maximum doping amount (incorporation amount) in the negative electrode, and the maximum charging capacity is also determined thereby.

Accordingly, in order to improve the battery capacity of the lithium secondary battery, not only the charge capacity of the carbon material used for the negative electrode is improved, but also the discharge capacity at that time is made closer to the charge capacity to minimize the irreversible capacity. It is necessary.

For these reasons, in recent years, when a non-graphitizable carbon material which is a porous material having a large specific surface area is used,
It was reported that a battery capacity exceeding the theoretical amount when using the graphite was obtained. This non-graphitizable carbon material is generally produced by carbonizing an organic compound containing carbon as a main component, and developing an array structure of carbon atoms by heat treatment. Then, the final characteristics of the non-graphitizable carbon material are controlled by the heat treatment in the post-process, so that the battery capacity (discharge capacity and charge capacity) greatly depends on the heat treatment temperature.

Further, in the case where a non-graphitizable carbon material using a furfuryl alcohol resin as a raw material is used for a negative electrode of a lithium secondary battery, the relationship between a heat treatment temperature and a battery capacity is
The charge capacity reaches its maximum value at 800 ° C. where the heat treatment temperature is relatively low.
It is reported that the maximum value is obtained at 100 ° C. Therefore, even if the heat treatment is performed at 800 ° C., the entire charge amount is not consumed for discharging, and the irreversible capacity remains. In practice, the heat treatment temperature of 1100 ° C., which has the maximum discharge capacity, is adopted, so that the battery is used in an insufficiently charged state.
That is, in a conventional lithium secondary battery using a carbon material as a negative electrode, increasing the charge capacity decreases discharge efficiency (a value obtained by dividing the discharge amount by the charge amount), and reduces the discharge amount.

However, in a lithium secondary battery, the mechanism of the electrode reaction such as how lithium ions are doped and dedoped in the above non-graphitizable carbon electrode has not been established yet. As described above, the cause of the irreversible capacity in which the charge capacity and the discharge capacity do not match has not been clarified.

Therefore, the present inventors have studied the behavior of non-graphitizable carbon used as a negative electrode of a lithium secondary battery, and minimized the irreversible capacity by discharging the charge amount as much as possible while improving the charge capacity. Intensive research was conducted with the aim of obtaining a lithium secondary battery with excellent battery capacity. as a result,
It has been found that when the amount of the organic solvent adsorbed on the non-graphitizable carbon serving as the negative electrode is reduced, the discharge efficiency does not decrease so much even when the charge capacity is increased. That is, as a result of studying the relationship between the physicochemical properties of non-graphitizable carbon and the charge / discharge capacity when using it, the organic solvent used as the electrolyte of the lithium secondary battery was converted to non-graphitizable carbon. The amount of adsorption depends on the battery capacity,
In particular, it has been found that the discharge capacity is significantly affected.
That is, it was found that when the amount of the organic solvent adsorbed on the non-graphitizable carbon was large, the discharge capacity was reduced.

[0012]

The lithium secondary battery of the present invention has been made based on the above findings, and has a positive electrode, a negative electrode and a negative electrode.
And a lithium secondary battery composed of the electrolyte solution, before
The pore diameter of the pores on the surface of the carbon anode material constituting the anode is
It has pores that allow lithium ions in the electrolyte to pass through and have a diameter that does not allow the organic solvent in the electrolyte to pass substantially.
Characterized by using fine particles of non-graphitizable carbon . Further, the non- graphitizable carbon fine particles have
There are pores that can adsorb the organic solvent in the electrolyte,
Pyrolytic carbon is deposited on the surface of
As a result, the pores are doped with lithium ions in the electrolyte and
Dedopable and organic solvent in electrolyte
Not be able to be adsorbed, and the average particle size is reduced to less than half
The adsorption amount of the organic solvent in the electrolytic solution at the time of
4.9% by weight, adding a binder and a pasting agent
In addition, it is characterized by being press-molded.

Further, a table of the finely graphitizable carbon fine particles is provided.
The pyrolytic carbon deposited on the surface is the pyrolysis of pyrolytic hydrocarbons.
It is characterized by being deposited by a method. Ma
Further, the thermally decomposable hydrocarbons include toluene, benzene,
Tan, 2,4-xylenol, quinoline, creosote
Or any one of the following. In the electrolyte
The solvent is propylene carbonate and dimethoxyethane.
Wherein the supporting electrolyte in the electrolyte is:
It is characterized by being lithium perchlorate.

Generally, the non-graphitizable carbon is a porous material having many pores and has an extremely large specific surface area.
Such surface activity takes up a lot of lithium ions,
It is considered to be the main cause of enhancing the electrocatalytic ability. And
This also increases the amount of adsorption of the organic solvent used as the electrolyte. However, as described above, when the amount of organic solvent adsorbed is large, the discharge capacity is reduced. To prevent this, it is necessary to reduce the amount of adsorbed organic solvent as much as possible while maintaining the porous property with a large specific surface area. There is.

On the other hand, the diameter of the pore inlet of the non-graphitizable carbon is determined by depositing carbon at the inlet of the pore by a pyrolysis method of a pyrolytic hydrocarbon as in the production process of molecular sieve carbon. Can be adjusted. Therefore, by selecting the optimum treatment conditions in the pyrolysis method for pyrolytic hydrocarbons, it is possible to make the diameter such that lithium ions can freely enter and exit, but the organic solvent cannot enter the pores. In this case, the pore volume of the non-graphitizable carbon does not substantially change.

That is, by depositing carbon generated by thermal decomposition at the entrance of the pores and narrowing the entrance diameter of the pores,
The adsorption amount of the organic solvent is substantially zero without changing the pore volume in which lithium ions are doped , that is, the amount of
Solvent can be made apparently impossible to adsorb .

[0017]

[Operation] According to the above configuration, the non-graphitizable carbon fine particles serving as the carbon negative electrode material have a pore entrance diameter in which lithium ions can pass and an organic solvent having a larger molecular size than the lithium ions can pass. Since the diameter is impossible, lithium ions can penetrate into the pores of the non-graphitizable carbon fine particles, but the organic solvent cannot substantially penetrate, and the organic solvent causing a decrease in discharge capacity is apparent. Adsorption becomes impossible . As described above, since only lithium ions can enter the pores of the non-graphitizable carbon, the discharge efficiency does not decrease so much, and the discharge amount can be increased as the charge amount is increased. to this
Therefore, the discharge capacity can be made closer to the charge capacity, and
Dramatically improve the battery capacity of rechargeable lithium batteries
it can.

[0018]

【Example】 Hereinafter, embodiments of the present invention will be described. Positive, negative
Lithium secondary battery of this embodiment composed of electrodes and electrolyte
Contains lithium ions, for example, as shown in FIG.
Of the separator 11 impregnated with an organic solvent as an electrolyte
A positive electrode 12 is disposed on one side and a negative electrode 13 is disposed on the other side.
In addition, the periphery of these cans, the cap 15, and the
A package for fixing the can 14 and the cap 15 in an insulated state
Formed in a coin shape so as to be covered with the ring 16.
You.

The negative electrode used in the lithium secondary battery of the present embodiment is obtained by processing a negative electrode material made of non-graphitizable carbon which has been subjected to a specific treatment into a desired shape. First, the negative electrode material was manufactured, and a confirmation test of a pore entrance diameter was performed. This is shown in Example 1.

Example 1 A phenol resin (PGA-4560 manufactured by Gunei Chemical Industry Co., Ltd.) was used as a raw material of non-graphitizable carbon as a negative electrode material, which was carbonized at 550 ° C., and then 800 ° C. in a nitrogen stream.
For 1 hour to obtain non-graphitizable carbon. Next, the non-graphitizable carbon was pulverized for 10 minutes by a vibration ball mill using alumina balls. As a result of measuring the average particle size of the obtained non-graphitizable carbon particles with a laser scattering light diffraction type particle size distribution analyzer, it was 6.2 μm.

Next, as a treatment for adjusting the pore entrance diameter, the non-graphitizable carbon fine particles were heated to 700 ° C., and nitrogen gas saturated with toluene at 25 ° C. was supplied at a rate of 3 liters per minute. After flowing for 1 hour, carbon generated by thermal decomposition of toluene was deposited on the surface.

Further, the non-graphitizable carbon particles after the above treatment are crushed again, and the pore volumes before and after the crushing and the adsorption amounts of propylene carbonate (PC) and dimethoxyethane (DME), which are the solvent of the battery, are measured. And each is measured,
It was confirmed that the pore inlet diameter was reduced by performing the pore inlet diameter adjusting process. Table 1 shows the results. The re-grinding time was 2 hours and 20 hours.

[0023]

[Table 1]

As shown in Table 1, when the non-graphitizable carbon particles having been subjected to the pore inlet diameter adjustment treatment on the surface of the fine particles having a particle diameter of 6.2 μm were crushed again, the average fine particle diameter was halved in 2 hours of the crushing time. It became 3.1 μm and further became 1.6 μm in 20 hours. In each case, the pore volume hardly changed by re-milling, but it can be seen that the adsorption amounts of PC and DME increased. In other words, before re-grinding, the pore inlet diameter is
That is PC and DME is focused on can not pass through diameter that the actual quality, although PC and DME could not penetrate into the pores,
It is understood that a new inlet was created when the average particle size was reduced to less than half by re-grinding, so that PC and DME could enter the pores. And at this time
The adsorption amount of PC and DME is 2.5 to 24.9% by weight.
Was.

Next, a test cell (evaluation cell) using the above-described negative electrode material was manufactured, and the performance of the negative electrode material of the present invention was evaluated by a charge / discharge capacity test.

FIG. 1 shows the structure of a test cell, 1 is an electrode using the carbon anode material of the present invention, 2 is a lithium electrode used as a counter electrode, 3 is a separator between both electrodes, 4 is an electrolyte, and 5 is a lithium electrode. Reference electrode.

The negative electrode material 1 was used as a binder in an amount of 10% by weight with respect to the non-graphitizable carbon particles having been subjected to the pore diameter adjustment process.
Of polyvinylidene fluoride from
After adding methylformamide to form a paste, it was pressed into a coin having a diameter of 10 mm and a thickness of 0.5 mm and used.

In the electrolyte, an organic solvent consisting of a 1: 1 mixed solution of propylene carbonate and dimethoxyethane was used.
As the supporting electrolyte, one to which 1.0 mol / l of lithium perchlorate (LiClO ₄ ) was added was used.

FIG. 2 is a conceptual diagram showing a change in current potential when a charge / discharge test is performed on the manufactured test cell. In addition,
Strictly speaking, in this test cell, the carbon electrode is a positive electrode, and lithium doping of the carbon electrode is discharge, but for convenience, this process is called charging for the actual battery, and conversely. The process of extracting lithium from the carbon electrode is referred to as discharge.

As shown in FIG. 2, charging was first performed. The initial potential of the negative electrode before energization is about 1.5 V with respect to the lithium reference electrode, and the current density is 0.53 mA / c.
When current supply was started at a constant current of m ² , the electrode potential gradually decreased. When the electrode potential reached 0 mV, the power was turned off when the current density was reduced by switching from the constant current to the constant potential, and the charging was terminated when the potential recovery was about 20 mV or less after a pause of 2 hours.

After a pause of 2 hours, discharge was performed. The discharge starts with a constant current of 0.53 mA / cm ² ,
When the potential reached 1.5 V, the potential was switched to the constant potential, and when the current density became 0.05 mA / cm ² or less, the discharge was terminated. Table 2 shows the measurement results of the charge / discharge capacity and the like at this time. The charge / discharge capacity is a capacity per 1 g of carbon,
The same applies to the following embodiments.

Example 2 A furan resin (VF-manufactured by Hitachi Chemical Co., Ltd.) was used as a raw material.
307), which was carbonized at 600 ° C.
Heat treatment at 800 ° C for 1 hour in flow to obtain non-graphitizable carbon
Was. Then, in the same manner as in Example 1, pulverization and pore inlet diameter adjustment
The electrode was prepared, and a charge / discharge test was performed. This
Of PC and DME adsorption amount, charge / discharge capacity, etc.
Table 2 shows the results.

EXAMPLE 3 Coconut garnish was used as a raw material, and this was washed with dilute hydrochloric acid.
To remove alkali, wash with water and dry.
Heat treatment at 800 ° C for 1 hour in air stream to obtain non-graphitizable carbon
Was. Then, in the same manner as in Example 1, pulverization and pore inlet diameter adjustment
The electrode was prepared, and a charge / discharge test was performed. This
Of adsorption amount of PC and DME, charge / discharge capacity, etc.
Table 2 shows the results.

[0034]

[Table 2]

Comparative Examples 1 to 3 Electrodes were prepared in the same manner as in Examples 1 to 3, except that the pore inlet diameter adjusting treatment was not performed, and a charge / discharge test was performed. Table 3 shows the measurement results of the amounts of PC and DME adsorbed and the charge / discharge capacity at this time.

[0036]

[Table 3]

From the results shown in Tables 2 and 3, a pore inlet diameter adjusting process is performed to narrow the pore inlet diameter, and the pore inlet diameter is adjusted so that lithium ions can pass therethrough and that A diameter through which the organic solvent cannot substantially pass, that is, charging and discharging are possible by passing lithium ions, and it is possible to prevent the organic solvent that causes a decrease in discharge capacity from entering the pores and being adsorbed. With such a diameter, the adsorption amount of the organic solvent to the non-graphitizable carbon that becomes the negative electrode is substantially zero ,
By making the organic solvent in the liquid apparently impossible to adsorb , the discharge capacity can be improved and the irreversible capacity can be minimized.

Example 4 A phenol resin was carbonized at 550 ° C.
Heat treatment was performed at 00 ° C. for 1 hour, and then pulverized to obtain non-graphitizable carbon fine particles. Next, as a treatment for adjusting the pore entrance diameter on the surface of the carbon fine particles, carbon precipitation treatment was performed in the same manner as in Example 1 using toluene, benzene, and butane as the thermally decomposable hydrocarbon vapor. . Next, a performance test was performed using the test cell of FIG. Table 4 shows the results.

[0039]

[Table 4]

Example 5 In order to adjust the pore entrance diameter of the non-graphitizable carbon fine particles obtained in the same manner as in Example 4, the deposition treatment was carried out using a liquid pyrolytic hydrocarbon as 2,4-xylenol, quinoline, Performed using creosote. That is, the above-mentioned hydrocarbon was impregnated and heat-treated at 900 ° C. for 1 hour. The amounts added in the carbon deposition treatment were 10% by weight, 5% by weight, and 15% by weight, respectively.
% By weight.

Next, a performance test was performed using the test cell of FIG. Table 5 shows the results.

[0042]

[Table 5]

Comparative Example 4 In the same manner as in Example 4, hardly graphitizable carbon fine particles were obtained. An electrode was prepared in the same manner as in Example 1 except that the treatment for adjusting the pore entrance diameter on the surface of the carbon fine particles was not performed, and PC and DM were prepared.
Table 6 shows the measurement results of the adsorption amount of E, charge / discharge capacity, and the like.

[0044]

[Table 6]

The treatment for adjusting the pore entrance diameter on the surface of the carbon fine particles was carried out in Example 4 using a thermally decomposable hydrocarbon in the form of vapor such as toluene, benzene and butane. In Example 5, xylenol and quinoline were used. And the use of liquid pyrolytic hydrocarbons such as creosote, etc., in both cases, the PC and DME adsorption amounts were almost zero compared to Comparative Example 4, the charge amount was large, and the discharge amount was significantly large. became.

Example 6 Next, a lithium secondary battery for testing was assembled as shown in FIG.
An in-mold was fabricated and its performance was tested. This lithium two
The following cell, the separator 11 was impregnated with an organic solvent containing lithium ions as the electrolyte, the separator 1
1 on both sides, a positive electrode 12 containing LiCoO ₂ as a main component;
A negative electrode 13 consisting of non-graphitizable carbon was opposed, these surroundings, can body 14, Tsu covered <br/> the cap 15 and the can body 14 and the cap 15 in the packing 16 fixed in an insulated state It is a thing.

As the negative electrode 13, the carbon electrode prepared in Example 1 was used. The positive electrode 12 has 10 g of lithium cobalt oxide.
1g of graphite, 1g of polytetrafluoroethylene
, And mix well, then take 2 g and take 10 mm in diameter.
What was press-molded was used. The separator 11 was a porous film of polypropylene. PC + D for electrolyte
ME (1: 1) obtained by adding LiClO ₄ to a concentration of 1.0 mol / l as a supporting electrolyte was used.

Using the above lithium secondary battery, charging was performed at a constant current of 0.53 mA / cm ² until the voltage reached 3.5 V, and then discharge was performed at the same current density with a discharge end voltage of 1.0 V. Was done. This operation was repeated to measure the change in charge / discharge capacity. Table 7 shows the results.

[0049]

[Table 7]

In the present invention, the shape of the lithium secondary battery itself, for example, the shape of the can 14 and the cap 15,
The size, the material used for the positive electrode 12, the type of organic solvent constituting the electrolyte, and the like are not particularly limited.
It can be arbitrarily selected according to various conditions such as a use state of the lithium secondary battery and a battery capacity.

Comparative Example 5 Using the carbon electrode prepared in Comparative Example 1 as a negative electrode, a coin-type lithium secondary battery was prepared in the same manner as in Example 6, and the results of repeatedly measuring the change in charge / discharge capacity are shown in Table 8. Show.

[0052]

[Table 8]

Example 6 is a battery using a carbon anode treated to adjust the pore entrance diameter on the surface of carbon fine particles according to the present invention, and has a larger charge / discharge as compared with Comparative Example 5 not treated. It has a capacity and excellent repetition charge / discharge characteristics.

[0054]

As described in the foregoing, the lithium secondary battery of the present invention, the pore inlet diameter of non-graphitizable carbon was used as a carbon negative electrode material, lithium ions in the electrolytic solution of a lithium secondary battery The diameter is such that the organic solvent used in the lithium secondary battery is adsorbed on the non-graphitizable carbon of the negative electrode, so that the discharge capacity can be reduced. Can be prevented, and at the same time, the charge capacity can be increased, the discharge efficiency can be increased, and the battery capacity of the lithium secondary battery can be significantly improved.

[Brief description of the drawings]

FIG. 1 is a sectional view of a test cell manufactured in an example and a comparative example.

FIG. 2 is a conceptual diagram of a change in current potential during a charge / discharge test of a test cell manufactured in Example 1.

FIG. 3 is a cross-sectional view illustrating an example of a lithium secondary battery.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Electrode using the carbon negative electrode material of this invention, 2 ... Lithium electrode used as a counter electrode, 3 ... Separator, 4 ... Electrolyte solution, 5
... Reference electrode, 11 ... Separator, 12 ... Positive electrode, 13 ...
Negative electrode, 14 ... can, 15 ... cap, 16 ... packing

────────────────────────────────────────────────── ─── Continuing on the front page (72) Akira Wakaizumi 3054-3 Shimokurosawa, Takane-machi, Kita-Koma-gun, Yamanashi Prefecture Inside Nippon Sanso Corporation (58) Field surveyed (Int.Cl. ⁶ , DB name) 4/58 H01M 10/40

Claims (8)

(57) [Claims] 1. A lithium secondary battery comprising a positive electrode, a negative electrode, and an electrolytic solution , wherein a carbon negative electrode material constituting said negative electrode is provided.
In addition, the pore inlet diameter of the surface, the lithium ions in the electrolyte solution can pass, and the organic solvent in the electrolyte solution is substantially non- graphitizable carbon having pores of a diameter that is substantially impermeable. Fine particles
A lithium secondary battery using : 2. The fine particles of non-graphitizable carbon, wherein
There are pores in the part that can adsorb the organic solvent in the electrolyte,
Pyrolytic carbon is deposited on the surface of the graphitizable carbon,
As a result, the pores are doped with lithium ions in the electrolyte.
And undoped, and the organic solvent in the electrolyte is
2. The lithium secondary battery according to claim 1, wherein the lithium secondary battery is apparently not adsorbable . 3. The method according to claim 1, wherein the non-graphitizable carbon fine particles are average particles.
Absorption of organic solvent in electrolyte when crushed to less than half the diameter
The lithium secondary battery according to claim 1 or 2, wherein the Chakuryou is 2.5 to 24.9 wt%. 4. The method according to claim 1, wherein the fine particles of the non-graphitizable carbon are a binder.
And press-molding
The lithium secondary battery according to any one of claims 1 to 3, wherein : 5. The method according to claim 5, wherein the fine particles of non-graphitizable carbon are deposited on the surface of the fine particles.
The released pyrolytic carbon is obtained by pyrolysis of pyrolytic hydrocarbons.
3. The composition according to claim 2, wherein the substance is deposited.
Lithium secondary battery. 6. The thermally decomposable hydrocarbon is toluene,
Senzen, butane, 2,4-xylenol, quinoline,
6. The method according to claim 5, wherein the material is any one of Leosote.
The lithium secondary battery according to the above. 7. The organic solvent in the electrolyte is propylene.
1: 1 mixture of carbonate and dimethoxyethane
The method according to any one of claims 1 to 6, wherein
Lithium secondary battery. 8. The supporting electrolyte in the electrolyte is perchloric acid.
8. The method according to claim 1, wherein said lithium is lithium.
The lithium secondary battery according to claim 1.