JP3525710B2 - Secondary battery and its positive electrode active material - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a secondary battery and its positive electrode active material.

[0002]

2. Description of the Related Art Conventionally, various metal oxides have been used as a positive electrode active material for secondary batteries. However, most of them have a low electron conductivity, so that the carbon oxide is less than the metal oxide. A conductive agent made of powder is added. By adding a conductive agent, electron conductivity is given between individual metal oxides to accelerate the reduction reaction.

[0003] Here, one of the causes that affect the battery performance.
One problem is the mixed state of the metal oxide and the conductive agent. That is, how uniformly and frequently the metal oxide powder and the carbon material fine powder are in contact with each other. When the contact strength is reduced, a portion where electrons are not sufficiently transferred and supplied to the positive electrode active material is generated, and as a result, an active material that remains unreacted is generated, and thus the utilization rate of the positive electrode active material is reduced.

To solve the above problem, for example,
Japanese Patent Application Laid-Open No. 61-214362 proposes a positive electrode active material in which fine graphite powder is layered on the surface of manganese dioxide particles, and Japanese Patent Publication No. 7-36332 discloses a metal oxide powder and an artificial graphite powder. The particle size ratio of 10 ^-1 to 10 ^-5 ,
A positive electrode active material has been proposed in which the coverage of the carbon material covering the metal oxide is 0.5 to 15%.

Further, in Japanese Patent Laid-Open No. 9-92265,
15 μm or more of the apparent surface of the metal oxide is 0.01 μm
A positive electrode active material composed of a positive electrode active material made of a carbon material having a specific surface area of 150 μm ² / g or more and covered with a thickness of 0.3 μm and carbon powder constituting a conductive agent interposed between the positive electrode active materials has been proposed. ing.

[0006]

However, according to the study by the present inventors, in the secondary battery using the positive electrode active material of each of the above publications, good positive electrode activity is obtained at low load and normal load. Although the utilization factor of the substance is shown, it has been found that the battery characteristics such as the capacity are insufficient under a high load condition such as a current density of about 4.0 mA / cm ² . This is presumed to be due to the following reasons.

That is, since the metal oxide powder is coated with the conductive agent powder, the conductivity is made uniform throughout the metal oxide. Therefore, the utilization efficiency of the active material is good and the load is high and the capacity is high. . However, since there is an interface between the particles of the individual metal oxide powders, this becomes resistance when the load is high, and the utilization efficiency is reduced, and the capacity is also reduced.

When the secondary battery is used in, for example, an electric vehicle or the like, it is essential to improve the performance of the secondary battery when the load is high, because a large capacity battery characteristic is required under both high load and low load. is there. Therefore, in view of the above points, the present invention has an object to provide a positive electrode active material for a secondary battery that achieves both good capacity under low load and high load.

[0009]

Means for Solving the Problems The inventors of the present invention can improve the utilization efficiency of an active material by coating the surface of individual metal oxide particles with a coating conductive agent as described above when the load is low. Attention was paid to maintaining a good capacity and further adding another conductive agent (hereinafter referred to as an added conductive agent) to form a conductive path of electrons at the time of high load to improve the battery capacity. In addition, the influence of the positive electrode active material on the electrode formation was also considered.

Generally, the coating of metal oxide with a conductive agent is
It is carried out by using a method called so-called mechano-fusion (mechano-fusion) in which both are mixed by mechanical energy such as compression and shearing and the plurality of particles are surface-fused. In the coating by the mechano treatment, the coating conductive agent needs to be fine particles smaller than the active material particles (that is, particles having a large specific surface area) above a certain level.

The present inventors examined the coating by this mechano treatment. The result is shown in FIG. FIG. 3 shows metal oxide particles after being coated with a coating conductive agent with respect to the amount of the coating conductive agent added to the metal oxide (for example, 0.5 wt% to 5.5 wt% as shown in the figure). It shows the transition of the specific surface area of (mixed powder). Here, the processing time on the horizontal axis is the time required for the above mixing.

The specific surface area of the conductive agent after coating decreases with the treatment time, but the reduction saturates at a certain time. Further, the specific surface area of the conductive agent after coating increases as the amount of the coating conductive agent added increases. In forming a coated metal oxide into an electrode, the particles are bound by a binder to form a positive electrode. However, the present inventors have found that the coating conductive agent is added to a certain extent (for example, 5 wt% or more). It has been found that above the above, the specific surface area of the conductive agent after coating becomes excessive (for example, larger than 40 m ² / g), and peeling or chipping of the electrode easily occurs. Here, the amount of the binder may be increased, but the battery capacity is reduced accordingly.

Therefore, if the specific surface area of the added conductive agent is the same as or larger than that of the coated conductive agent, the added amount of the coated conductive agent is substantially increased when the added conductive agent is added. As described above, the specific surface area of the conductive agent after the coating may become excessive, and the above-mentioned peeling or chipping may occur in the electrode. Therefore, for making electrodes,
The added conductive agent preferably has a specific surface area smaller than that of the coated conductive agent. Further, by making the specific surface area of the added conductive agent smaller than that of the coated conductive agent, the particle size of the added conductive agent can be increased,
It is considered that it is easy to form an electron conductive path by interposing between the metal oxide particles.

Based on the above-mentioned examination results, the following technical means have been adopted. That is, according to the invention described in claim 1, a large number of metal oxide particles constituting the active material main body, a first conductive agent coating the surface of each particle of the metal oxide, and each particle of the metal oxide. Lithium containing a second conductive agent interposed between and having a specific surface area smaller than that of the first conductive agent
The positive electrode active material of the secondary battery is featured.

As a result, the capacity under low load can be improved by the first conductive agent, and at the time of high load, the conductive path of electrons is secured by the second conductive agent interposed between the metal oxide particles. Since the active material (metal oxide) around it is used, utilization efficiency can be improved and good capacity can be realized. Further, the second conductive agent has a first specific surface area
Since it is smaller than the conductive agent of No. 3, the specific surface area of the conductive agent after coating can be prevented from becoming excessive, and peeling or chipping can be prevented in forming an electrode.

Therefore, according to the present invention, it is possible to provide a positive electrode active material for a secondary battery, which has good capacity at low load and high load, and which can be favorably made into an electrode. Furthermore, in order to achieve the above effect more stably, in the invention according to claim 1, the addition amount Awt% of the first conductive agent to the metal oxide and the total addition amount Bwt% of both conductive agents to the metal oxide are provided. Is 0.5 wt% ≦ A ≦
4.5 wt%, 2.5 wt% ≤ B ≤ 14.0 wt%, and when 0.5 wt% ≤ A ≤ 1.5 wt%, 2.5 wt% ≤ B ≤ 14.0 wt%, 1.5 wt % <A
When ≦ 2.5 wt%, 3.5 wt% ≦ B ≦ 14.0 wt
%, 2.5 wt% <A ≤ 3.5 wt% 4.0 wt
% ≦ B ≦ 14.0 wt%, 3.5 wt% <A ≦ 4.5 w
A positive electrode active material of a secondary battery having a relationship of 4.5 wt% ≦ B ≦ 14.0 wt% at t% , comprising a metal oxide,
The specific surface area of 250m ² / g~1500m ² / g first
Multiple conductive particles are mixed to convert multiple particles into mechanical energy.
Each particle of the metal oxide is formed by surface fusion according to
It is assumed that the child surface is covered with the first conductive agent.
It is characterized by In the invention according to claim 2 ,
The positive electrode active material of the secondary battery according to claim 1 , wherein the second
Of the conductive agent has a specific surface area, characterized in that a 1m ² / g~200m ² / g. In the invention according to claim 3, claim
In a positive electrode active material for a secondary battery according to 2, the second conductive agent has a specific surface area, characterized in that 1m ² / g~25m ² / a is. In the invention according to claim 4 , claims 1 to 3
In the positive electrode active material of the secondary battery according to any one of the above, the specific surface area of the particles of the metal oxide coated with the first conductive agent is 40 m ² / g or less.

[0017]

[0018]

[0019]

[0020]

Further, as in the invention described in claim 5, the secondary battery provided with the positive electrode active material according to any one of claims 1 to 4 is excellent at low load and high load. It is possible to provide a secondary battery having a positive electrode that is compatible with various capacities and is less likely to cause peeling or chipping.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION In the present embodiment, a Li secondary battery will be described below as a secondary battery. In this embodiment, lithium manganese oxide, lithium nickel oxide, lithium cobalt oxide, manganese oxide, or the like can be used as the metal oxide forming the positive electrode active material body.

The coating conductive agent (first conductive agent) for coating the surface of each particle of the metal oxide to make the conductivity of electrons uniform in the metal oxide has a specific surface area of 250 m ² / g to 1
It is possible to use Ketjenen black or acetylene black, which is 500 m ² / g. In order to further improve the capacity, the specific surface area is 1000 m ² / g to 150
It is more preferably 0 m ² / g (JP-A-9-92).
(See FIG. 4 of Japanese Patent No. 265).

Further, the specific surface area which is further added to the coated metal oxide and is present between the particles of the metal oxide to form a conductive path of electrons is higher than that of the coated conductive agent (first conductive agent). as small additive conductive agent (second conductive agent) can be used carbon powder or the like is specific surface area of 1 m ² / g to 200 m ² / g. In order to improve the electrode density,
And more preferably a specific surface area of 1m ² / g~25m ² / g approximately.

Hereinafter, as an example of the positive electrode active material, metal oxide
LiMn as a product₂O_Four(Average particle size 7 μm)
Average particle size of 0.03 μm and specific surface area of 127 as a covering conductive agent
0m ²/ G lion made Ketchen black (hereinafter,
(Abbreviated as KB) as an additive conductive agent
KS-6 (average particle size 3.4 μm, specific surface area 21.3 m)
²/ G), and KS-15 manufactured by the same company (average particle size 7.5μ)
m, specific surface area 12.8 m²/ G)
It The present embodiment is not limited to this example.

LiMn₂O_FourOn the surface of each particle of
The mechano treatment to cover is LiMn₂O _FourA certain amount of KB
The mixed powder was mixed using the film forming device shown in FIG.
It was This film forming device has a rotary drive with an internal space 10.
And a fixed shaft 2 inside the rotary drum 1.
Semi-circular pressing extending near the inner peripheral surface of the rotating drum 1.
A first arm 4 having a shearing head 3 and the first arm 4
It is fixed to the fixed shaft 2 at a predetermined angle behind the rotation of the
A second arm having a claw 5 extending near the inner peripheral surface of the drum 1.
It is composed of 6 and.

The above mixture is placed in the internal space of the film forming apparatus.
Put the powder, and rotate the rotary drum 1 at a predetermined rotation speed for a predetermined time (processing
Time) to rotate the inner peripheral surface of the rotating drum 1 and the pressing shear head.
A pressing shearing force is applied to the blade 3 and then scraped off with the nail 5.
And then mixed to form LiMn ₂O_FourK on each particle surface of
B was coated. With the KB obtained by the above mechano-processing,
Capped LiMn₂O_Four, KS-6 or KS-1
5 as a binder PVDF (polyvinylidene fluoride)
Added together, and further solvent (N-methyl-2-pyrrolidone
Etc.) and kneaded to form a paste. Where the binder
Is set to several% (for example, about 3%).

The obtained paste was coated on an Al foil current collector by a doctor blade method, and then dried,
The electrode (positive electrode) was created by press molding. This electrode was punched into a disc shape, further vacuum dried, and then carried into a dry box to manufacture a coin battery. MCMB (mesophase carbon microbeads) made by Osaka Gas is used as the counter electrode (negative electrode), Tapirus 25 μm made by Tonen Kagaku is used as the separator, and 1 M LiPF ₆ / EC (50) is used as the electrolytic solution.
An electrolytic solution was used in which 1M LiPF ₆ was dissolved in a solvent in which DME (50), that is, EC (ethylene carbonate) and DME (dimethoxyethane) were mixed by 50% by volume.

The evaluation conditions were that charging was 4.2 V × 5 h, and 1.
Discharge is 3.0V under conditions of 0mA / cm ² and CC / CV
Cut and CC. The discharge current density of 4.0 is shown in the table of FIG.
In the case of mA / cm ² , that is, at the time of high load, the positive electrode capacity ratio in various positive electrode active materials of the present example (hereinafter, referred to as capacity ratio)
Indicates. The chart in FIG. 2 shows the KB for LiMn ₂ O ₄ .
The amount ratio and the total amount of the conductive agent when KS-15 or KS-6 is added (each wt%) are variously changed, and the capacity ratio is shown. Here, the above KB amount is the addition amount Awt% of the coating conductive agent (first conductive agent) to the metal oxide.
And the total amount of the above conductive agents corresponds to the total amount Bwt% of both conductive agents added to the metal oxide.

The above KB amount is the KB amount when the total of the LiMn ₂ O ₄ amount and the KB amount is 100 wt%,
0 wt%, 0.5 wt%, 1.5 wt in the leftmost column of the chart
%, 2.5 wt%, 3.5 wt%, 4. It is shown as 5 wt%. The total amount of the conductive agent is the amount of LiMn ₂ O _{4, the} amount of KB, the amount of KS-15 (or the amount of KS-6), and the binder (P
VDF) is the total addition amount of KB amount and KS-15 amount (or KS-6 amount) when the total sum with VDF) is 100 wt%, and is 1.5 wt% to 10.0 wt% at the top of the chart. Are shown up to .

Here, the capacity ratio is such that the KB amount is 0 wt%, K
Since the capacity when 10 wt% of S-15 or KS-6 was added was a good capacity, the capacity at this time is shown as 1. Then, for example, when the amount of KB is 1.5 wt% and the total amount of conductive agent is 2.5 wt% (that is, the amount of KS-15 is 1.0 wt%), the capacity ratio is expressed as 1.04. From this chart, the one showing good capacity, ie, capacity ratio 1
The above (value surrounded by a broken line in FIG. 2) has a KB amount (A) of 0.5 wt% ≦ A and a total conductive agent amount (B) of
2.5 wt% ≦ B, further, from the total amount of conductive agent (B) to KB
The value B-A obtained by subtracting the amount (A) is B-A ≧ 0.5 wt.
It turns out that it becomes a thing of%. By the way, B-A <0.
In the case of 5 wt%, that is, when the amount of KS-15 or KS-6 was small, a good capacity ratio could not be obtained.

Also, when the capacity was examined at a low load (0.5 mA / cm ² ), when the total amount of the coating and the added conductive agent was 15 wt% or more, the amount of LiMn ₂ O ₄ decreased and the low load. It was found that this is not preferable because the capacity at that time decreases. Therefore, the total amount of the coating and the added conductive agent (total conductive agent amount Bwt%) is 2.5wt% ≦ B ≦ 14.0w.
It is desirable to be t%.

Further, as shown in FIG. 3 mentioned in the section of the solving means, it is desirable that the added amount of the coating conductive agent, that is, KB (coating conductive agent) is 5.0 wt% or less for forming an electrode. . In FIG. 3, the processing time on the horizontal axis is the mechano processing time (unit: minutes), and the vertical axis is LiMn ₂ O ₄ in KB.
Is a specific surface area (unit: m ² / g) of the mixed powder after coating with, and each plot mark corresponds to each addition amount of KB. When the amount of KB added is larger than 5.0 wt%, the specific surface area becomes larger than 40 m ² / g, so that peeling or chipping of the electrode easily occurs.

The following can be said based on the knowledge about the amount of each conductive agent added. That is, KB
Regarding (coating conductive agent), the addition amount (KB amount Awt%) to LiMn ₂ O ₄ is 0.5 wt% ≦ A ≦ 5.0 w
t% is preferable, and the total amount of the coating and the added conductive agent (total conductive agent amount Bwt%) is 2.5 wt% ≦ B ≦.
It is preferably 14.0 wt%.

As described above, as the positive electrode active material, a large number of particles of metal oxide constituting the active material body and the surface of each particle of the metal oxide are coated to uniformize the conductivity of electrons in the metal oxide. By using a conductive agent and an added conductive agent which is present between the particles of the metal oxide and has a specific surface area which forms a conductive path of electrons at high load and is smaller than the coating conductive agent, at low load and It is possible to provide a positive electrode active material of a Li secondary battery that has good capacity at the time of high load and that can be preferably formed into an electrode.

Here, it can be presumed that the mechanism capable of achieving both good capacity under low load and high load is as follows. The operation of the conductive agent in the positive electrode of FIG. 4 will be described with reference to FIG. In FIG. 4, 20 is an added conductive agent, 21 is a coated conductive agent, 22 is a positive electrode active material particle (hereinafter referred to as active material), and 23 is a current collector. Further, (a) shows the case where only the added conductive agent 20 is contained in the active material 22, (b) shows the case where only the coated conductive agent 21 is contained in the active material 22, and (c) shows the case where both the coating and the added conductive agents 20 and 21 are active. This is the case when it is contained in the substance 22.

At the time of low load, in the electrode of (a), since there are many gaps between the active materials 22, there is an active material 22 which is not conductive, and the capacity is not sufficient. In the electrode of (b), since the coating conductive agent 21 provides uniform conductivity, the utilization efficiency of the active material 22 is good and the capacity is high.
At the time of high load, in the electrode of (a), since the added conductive agent 20 is localized, a conductive path of electrons having very good conductivity is formed in that portion, and the active material 22 in the vicinity thereof is used.
Therefore, the capacity is high. In the electrode of (b), since there are interfaces between the individual active materials 22, it is considered that these interfaces serve as resistance to lower the utilization efficiency and lower the capacity.

On the other hand, since the electrode of (c) is a combination of the electrodes of (a) and (b), the utilization efficiency of the active material 22 can be increased at low load and high load, and good capacity can be obtained. Can achieve both. By the way, when the powder resistance was examined for each of the positive electrode active materials in which the compounding ratio in the diagram of FIG. 2 was changed, the results shown in FIG. 5 were obtained.

FIG. 5 is a graph showing the relationship between the total amount of coating and additive conductive agent added to the positive electrode active material (total amount of conductive agent) and the powder resistance of the positive electrode active material. The vertical axis indicates the powder resistance (Ω). The horizontal axis represents the total amount of conductive agent (wt
%), But from the left, 10.0 wt%, 2.5
The four cases of wt%, 4.5 wt% and 6.5 wt% are vertically separated by dotted lines.

Then, in the area of each total amount of conductive agent,
The value shown in the graph is the amount of KB (wt%)
The plots are plotted from left to right in order of increasing volume from the least. The plot of the total conductive agent amount of 10.0 wt% region is the KB amount of 0 wt%, that is, the mechano-unprocessed one. Further, in the figure, □ indicates that KS-6 and KB are included, and ⋄ indicates that KS-15 and KB are included. Therefore, for example, in the region where the total amount of conductive agent is 6.5 wt%, the leftmost square has a powder resistance of about 1.0 × 10 ^2.
Ω, which is 0.5 wt% of KB and is KS-6.
Is 6.0 wt%.

From FIG. 5, it can be seen that those having good capacity in the chart of FIG. 2 are in the region where the powder resistance is 1.0 × 10 ² Ω or less. In other words, the powder resistance is 1.
If it is 0 × 10 ² Ω or less, the load characteristics are improved.
It is considered that this is because the electronic conductivity of the active material was improved. As described above, the present embodiment has been described based on the above example. However, if each metal oxide, coating conductive agent, and added conductive agent described above in the present embodiment, the same effect as this example can be obtained. .

[Brief description of drawings]

FIG. 1 is a schematic sectional view showing a configuration of a film forming apparatus according to an embodiment of the present invention.

FIG. 2 is a table showing capacity ratios for various addition amounts of a conductive agent in a positive electrode active material.

FIG. 3 is a graph showing changes in the specific surface area of the mixed powder after coating with respect to the amount of the coating conductive agent added to the metal oxide in the mechano treatment.

FIG. 4 is an explanatory view showing the action of a conductive agent in the positive electrode.

FIG. 5 is a graph showing the relationship between the amount of conductive agent added and the powder resistance of the positive electrode active material.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Rotating drum, 2 ... Fixed shaft, 3 ... Press shearing head, 4
... 1st arm, 5 ... Claw, 6 ... 2nd arm, 10 ... Internal space, 20 ... Additive conductive agent, 21 ... Coating conductive agent, 22 ... Positive electrode active material particle, 23 ... Current collector.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-2-262243 (JP, A) JP-A-9-92265 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H01M 4/00-4/62 H01M 10/40

Claims (5)

(57) [Claims] 1. A large number of metal oxide particles constituting an active material main body, a first conductive agent coating the surface of each particle of the metal oxide, and a metal oxide interposed between the particles of the metal oxide, A positive electrode active material for a lithium secondary battery, comprising a second conductive agent having a specific surface area smaller than that of the first conductive agent, wherein the added weight Aw of the first conductive agent to the metal oxide is
t% and the total addition amount Bwt% of the both conductive agents to the metal oxide are 0.5 wt% ≦ A ≦ 4.5 wt% and 2.5 wt% ≦ B ≦
If the relationship is 14.0 wt% and 0.5 wt% ≤ A ≤ 1.5 wt%, 2.5 wt% ≤
When B ≦ 14.0 wt%, 1.5 wt% <A ≦ 2.5 wt%, 3.5 wt% ≦
When B ≦ 14.0 wt%, 2.5 wt% <A ≦ 3.5 wt%, 4.0 wt% ≦
When B ≦ 14.0 wt%, 3.5 wt% <A ≦ 4.5 wt%, 4.5 wt% ≦
B ≦ 14.0 wt%, which is a positive electrode active material for a secondary battery having a specific surface area of 250 m ² / g to 150 with the metal oxide.
A plurality of particles mixed with 0 m ² / g of the first conductive agent
By surface fusion with mechanical energy,
The surface of each particle of the metal oxide is protected by the first conductive agent.
A positive electrode active material for a secondary battery, which is characterized by being coated with 2. The second conductive agent has a specific surface area of 1 m ²
/ G to 200 m according to claim 1, characterized in that the ² / g
The positive electrode active material of the secondary battery according to. 3. The specific surface area of the second conductive agent is 1 m ²
/ G ~ 25m < ² > /, The positive electrode active material of the secondary battery according to claim 2 . 4. The specific surface area of the particles of the metal oxide coated with the first conductive agent is 40 m ² / g or less, according to any one of claims 1 to 3 . Positive electrode active material for secondary batteries. 5. A secondary battery comprising the positive electrode active material according to any one of claims 1 to 4 .