JPH02262243A - Positive electrode for lithium secondary battery and manufacture thereof - Google Patents

Abstract

PURPOSE:To stabilize discharge capacity at all times regardless of a cycle progress by fixing a carbon fine powder having the predetermined mean grain size ratio against the MnO2 grain of an active material to the surface of the MnO2 grain for forming a complex grain powder, and using the material so formed as a positive electrode. CONSTITUTION:For the MnO2 grain of an active material, a carbon material fine powder having a mean grain size ratio of 10<-1> to 10<-5> is fixed onto the MnO2 grain surface, thereby forming a positive electrode having a complex grain powder. Furthermore, a granulated graphite powder or an activated carbon powder as a conductive material is contained in the aforesaid positive electrode, in addition to the carbon material fine powder contained in the complex grain powder. Moreover, the carbon material fine powder in the aforesaid complex grain is carbon black or fibrous graphite. When the carbon black is used, the amount thereof fixed to the surface of MnO2 is taken at 2 to 5wt% for MnO2. Also, when the fibrous graphite is used, the amount thereof fixed to the surface of MnO2 is taken at 1 to 3wt.% for MnO2. In addition, the granulated graphite powder or the activated carbon powder is taken at 2 to 5wt.% for MnO2.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a lithium secondary battery having a high energy density using lithium (Li) as a negative electrode, particularly to a positive electrode and a method for manufacturing the same.

Conventional technology lithium batteries use manganese dioxide (MnO2) as the positive electrode.
Primary batteries using are well known. In addition, the development of rechargeable lithium secondary batteries is actively progressing.
MnO2 is also expected to be a positive electrode active material for this secondary battery that has excellent charge/discharge reversibility, capacity, and voltage. As is well known, the positive electrode active material is a powder consisting of MnO2 particles, and the MnO2 positive electrode for lithium batteries uses carbon material powder such as carbon black or graphite as a conductive agent, and fluorine such as polytetrafluoroethylene as a binder. It uses resin.

Then, press molding the powder mixture by mixing these,
It is made into a clay-like or paste-like form by adding a dispersion medium such as water, and is filled into an electrode plate core such as a stainless steel mesh and used as an electrode plate. In addition, in a battery using M n O2 as an active material,
Existing batteries include dry batteries, alkaline manganese batteries, and rechargeable lithium batteries, but depending on the application, the conductive agent may be carbon black such as acetylene black (AB), crystals such as granular graphite or fibrous graphite. Proposals have been made to use carbon material powder and the like. Regarding the mixing method, it is common to simply mix using a mixer, but there is also a method in which MnO2 particles and graphite particles, which are one of the conductive agents, are charged with opposite static charges, and the surface of the MnO2 particles is charged. A method of forming a graphite layer by adsorption to form a mixture has been proposed (Japanese Patent Application Laid-open No. 214362/1983). moreover,
A method of embedding and fixing fine carbon powder on the surface of metal oxide particles (Japanese Patent Application No. 62-27710) and a method of fixing fibrous graphite on the surface of MnO2 particles (Japanese Patent Application No. 62-22430)
No. 3) has been proposed. In particular, the main purpose of these proposals was to improve the utilization rate of active materials in primary batteries.

On the other hand, in the case of a lithium secondary battery, metal Li is used for the negative electrode, and its filling capacity is generally configured to be several times the capacity of the positive electrode. This is because the charging/discharging reaction of this battery includes a consumption reaction of the negative electrode Li, and the negative electrode is filled in advance in excess to ensure a long cycle life.

For example, if a lithium secondary battery is prototyped by combining a metal Li negative electrode with a positive electrode prepared by simply mixing MnO2 and carbon powder in a mixer, and then charged and discharged, the second
The capacity-cycle characteristics are shown as shown in the figure. As the cycle progresses, the charge/discharge capacity gradually decreases, and at a certain point (point A in the figure), the capacity decreases and reaches the end of its life. From this point of view, the point at which the life span is reached due to depletion of the negative electrode is considered to be point A in FIG. 2, and before that point, it is considered that the capacity decrease is due to the positive electrode.

Problems to be Solved by the Invention An object of the present invention is to provide a lithium secondary battery that always has a stable discharge capacity regardless of the progress of the cycle. An object of the present invention is to improve the positive electrode in order to suppress the decrease in charge and discharge capacity caused by the positive electrode as described above.

Means for Solving the Problems More preferably, the positive electrode has a composite particle powder in which carbon material fine powder having an average particle size ratio of 10-1 to 10-5 to the MnO2 particles of the active material is immobilized on the surface of the MnO2 particles. The positive electrode contains granular graphite powder or activated carbon powder as a conductive agent in addition to the carbon material fine powder in the composite particles.

Further, the carbon material fine powder in the composite particles is carbon black or fibrous graphite, and when carbon black is used, the amount of the carbon material fixed on the surface is preferably 2 relative to MnO2.
The amount should be at least 5% by weight, and when fibrous graphite is used, the amount fixed on the surface is preferably M
The content should be 1% by weight or more and 3% by weight or less based on nO2. Furthermore, the above-mentioned granular graphite powder or activated carbon powder preferably accounts for 2% by weight or more and 5% by weight or less based on MnO2.

Moreover, the manufacturing method of the above-mentioned positive electrode is as follows. First, the above-mentioned composite particles are made by stirring MnO2 powder and carbon material fine powder while dispersing them in a gas phase.
After adsorbing carbon fine powder onto O2 particles, this is treated with a high-speed airflow impact method (a method in which the powder is thrown into an airflow with a rotating body rotating at high speed and the powder particles are strongly collided with each other), It is produced by implanting and fixing carbon material fine powder onto the MnO2 surface. Further, when granular graphite powder or activated carbon powder is added to and mixed with the composite particle powder, it is preferably carried out by utilizing the above-mentioned charge adsorption effect.

With the above means, the problems of the prior art described above can be solved.

Function In the conventional MnO2 positive electrode, since the active material MnO2 itself has no electronic conductivity, current collection is performed by adding carbon powder such as acetylene black as a conductive agent. Further, the active material particles and the conductive agent particles are fixed with a binder and brought into strong contact by rolling the electrode plate, but this is merely mechanical contact.

For example, in the case of secondary batteries, since the battery only needs to be discharged once, performance can be brought out even with this current collection method. However, in the case of secondary batteries, charging and discharging are repeated, that is, L into the MnO2
Since the insertion and release of i is repeated, the MnO2 particles constantly expand and contract. Therefore, M
It is thought that the current collection network between the nO2 particles and the conductive agent particles becomes loose, resulting in a decrease in current collection efficiency and a decrease in capacity caused by the positive electrode.

The present invention aims to strengthen this current collection network, and first of all, it uses composite particles in which a part of a conductive agent is fixed to MnO2 particles to ensure current collection between the MnO2 particles and the conductive agent. It is something to do. Second, MnO2 in the positive electrode
A conductive agent other than the conductive agent fixed to the particles is added, thereby forming an auxiliary network and further enriching the current collection network.

Example Examples of the present invention will be shown below.

Example 1 Composite particles according to the present invention were prepared by drying M n 0
The two particles and the carbon material fine powder are given opposite electrostatic charges, and the carbon material fine powder is adsorbed onto the MnOx particles using the charge adsorption effect, and then the carbon material fine powder is introduced into a high-speed air flow to cause collisions between the particles. It is obtained by injecting and fixing raw material fine powder onto the surface of MnO2 particles. However, when the relative humidity of the gas phase in the charge adsorption process is 30% or more, the electrostatic charge disappears due to moisture, and the charge adsorption effect is not effective. Also, in the process of implanting carbon material fine powder onto the surface of MnO2 particles in high-speed airflow, if the relative humidity is 30% or more, 0,
Secondary aggregation occurred between particles of the same type, and uniform dispersion was not achieved. Therefore, in the above process, the atmosphere is adjusted to a relative humidity of 3
It is necessary to do this at less than 0%.

Furthermore, in the charged adsorption process, the adsorption effect was not established experimentally unless the average particle size ratio of the carbon material fine powder to the MnO2 particles was set to 10<-1> or less.

In addition, if carbon material fine powder with an average particle size ratio of 10-5 or less is used, the particles will aggregate together to form a particle group, so the average particle size ratio will actually become thicker (there will be almost no difference from what it became). Therefore, the above average particle size ratio is 10-1 to 10
It was found that a range of −5 is preferable.

Reliably fixing the carbon material fine powder to the MnO2 particles could not be achieved only by the electrostatic adsorption effect, but was achieved by utilizing mechanical energy by the high-speed air impact method, which is a subsequent process. Generally, when using this method, mechanical operations are controlled by considering the balance of specific gravity and particle size of each material in order to match the type of active material and conductive agent. For example, when carbon black, which is the immobilized carbon material fine powder according to the present invention, was studied, the rotational speed of the rotating body in the high-speed air impact method was 1500 to 15000 r.
A uniform immobilization state was obtained within the pm range. 1500
If the speed is below rpm, immobilization will be insufficient and the immobilization will be uneven and non-uniform. Meanwhile 15000 r
When the impact force is more than pm, the impact force is too strong, and the destruction of the MnO2 particles progresses, resulting in peeling of the material fine powder and the appearance of a new MnO2 surface on which the carbon material fine powder is not adsorbed, which has the opposite effect. In addition, when using fiber graphite as the fixed carbon material fine powder, as a result of study, it was found that
A uniform immobilization state was obtained in the range of 0.00 rpm. The above observation of the immobilized state was performed using an electron microscope.

Example 2 A study was conducted regarding the amount of immobilized carbon material fine powder fixed on the surface of Mn02 particles. Using AB as a conductive agent for immobilization, using the above-mentioned charge adsorption effect and high-speed air impact method,
The surface fixed amount is 1% by weight, 2% by weight, 3% by weight based on MnO2.
% by weight, 4% by weight.

A mixture consisting of seven types of composite particles of 5% by weight, 6% by weight, and 7% by weight was prepared. For further comparison, simply Mn
A conventional positive electrode mixture made by mixing O2 and AB in a mixer was also prepared. In the case of the conventional positive electrode mixture, as a result of preliminary studies, MnO
A battery containing 5% by weight of AB based on 2% showed relatively good battery characteristics, so this was used. Furthermore, polytetrafluoroethylene resin powder was added to each mixture as a binder in an amount of 5% by weight based on MnO2.

A button-shaped battery as shown in FIG. 3 was used as the test battery in this study. In Fig. 3, the positive electrode 3 is placed on a titanium net 5 fixed by spot welding inside the positive electrode case 4.
．． The positive electrode mixture containing 2 g of MnO2 was press-molded. And a polypropylene separator 6
, sealed together with a metallic lithium negative electrode 8 and an electrolyte 9 (1 mol/e LiAsF5 dissolved in a mixed solvent of propylene carbonate and ethylene carbonate) pressed to a sealing plate 7 via a polypropylene gasket 10, Diameter 20I
1111, the battery has a height of 1.6 mm. Additionally, since this battery is used to compare the characteristics of the positive electrode, the capacity of the negative electrode is filled to about four times the capacity of the positive electrode, so that the charging and discharging characteristics are not affected by the lack of the negative electrode. .

The charge/discharge test was conducted at a constant current charge/discharge of 2 A, with a charge end voltage of 3.8V and a discharge end voltage of 2. I set it to OV.

Figure 4 is a charge/discharge capacity-cycle characteristic diagram of each battery, plotted up to 100 cycles, and shows the characteristics of each battery with a conductive agent immobilized on MnO2 particles (shown as a solid line,
11:1% by weight, 12:2% by weight, 13:3% by weight, 1
4: 4% by weight, 15: 5% by weight, 16: 6% by weight, 1'
/near weight %) and the characteristics of a battery using a conventional positive electrode (indicated by a broken line 18). From this figure, it can be seen that immobilization of the conductive agent is effective in reducing the capacity reduction rate accompanying cycles. However, when looking at the characteristics of a battery with a surface-immobilized amount of 1% by weight, its capacity is small. That is, the active material utilization rate is low. In addition, both the active material utilization rate and capacity reduction rate improved as the amount of conductive agent increased, and the amount of surface immobilization increased by 5% by weight.
In batteries ranging from 7% by weight, both the utilization rate and the rate of capacity decline are excellent, but no significant difference can be seen.
In the case of AB, it can be said that a range of 2% by weight or more and 5% by weight or less for the surface fixed amount is practically effective.

Next, similar studies were conducted on carbon black other than AB, activated carbon granular graphite, fibrous graphite, etc. as conductive agents for immobilization. For carbon black, it has been found that the amount fixed on the surface is preferably in the range of 2M% or more and 5% by weight or less, as in the case of AB. For fibrous graphite, favorable results were obtained when the amount fixed on the surface was 1% by weight or more and 3% by weight or less, but the utilization rate was slightly lower than that of carbon black (on the contrary, if it exceeded 3% by weight, the utilization rate decreased This is thought to be due to the fact that the fibrous graphite covered the MnO2 surface too much, reducing its wettability to the electrolyte.Also, in the case of activated carbon granular graphite, the utilization rate was also low. (Also, no effect was obtained regarding capacity reduction.

This is presumed to be because the activated carbon 2-grain graphite is a hard carbon material and cannot flexibly respond to loosening of the positive electrode.

As described above, carbon black or fibrous graphite is suitable as the carbon material fine powder fixed on Mn0z.

Example 3 In order to form an auxiliary network to further enhance the current collection network by immobilizing carbon material fine powder to MnO2 shown in Example 2, a conductive agent was added in addition to the immobilized carbon material fine powder. Study was carried out.

Using a mixture consisting of composite particles in which 5% by weight of AB is immobilized with respect to MnO2, carbon black, fibrous graphite, activated carbon 9 granular graphite, etc. are each added in an amount of 3% by weight relative to MnO2 as a conductive agent for the auxiliary network. A mixture was prepared by mixing the mixture in a mixer, and batteries similar to those in Example 2 were constructed from each mixture and compared and studied. As a result, it was found that the addition of carbon black and fibrous graphite only increased the bulk, but the active material utilization rate and capacity reduction rate were almost the same as when they were not added, and were therefore ineffective. However,
When activated carbon and granular graphite were added and mixed, no effect was observed on the active material utilization rate, but an effect was observed on the capacity reduction rate.

Therefore, we investigated the amount of granular graphite added, which is effective as a conductive agent for the auxiliary network. Granular graphite was added to the above powder consisting of composite particles in which AB was immobilized in an amount of 5% by weight based on MnO2, respectively.
% by weight, 3% by weight.

Seven types of mixtures containing 4% by weight, 5% by weight, 6% by weight, and 7% by weight were prepared, and batteries similar to those described above were constructed for each mixture and a charge/discharge test was conducted.

FIG. 5 is a charge/discharge capacity-cycle characteristic diagram of each battery, plotted up to 100 cycles, and shows the characteristics of each battery to which granular graphite for the auxiliary network was added and mixed (19:1 weight % indicated by the solid line).

20:2% by weight, 21:3% by weight, 22:4% by weight, 2
3:5% by weight, 24:6% by weight, 25% by weight) and M
It shows the characteristics (indicated by a broken line 26) of a battery made only of composite particles in which 5% by weight of AB is immobilized on nO2. From this figure, it can be seen that the addition and mixing of the conductive agent for the auxiliary network is effective in reducing the rate of capacity decrease due to cycles. However, in the case of a battery in which the amount of added mixture was 1% by weight, little improvement was observed in terms of capacity reduction rate. In addition, the capacity reduction rate improves as the amount of added mixture increases, but 5
For batteries ranging from 7% to 7% by weight, the capacity reduction rate is excellent, but no significant difference can be seen. Therefore, the larger the amount of mixture added, the higher the bulk (considering that
It can be said that it is practically effective for the amount of granular graphite added to be in the range of 2% by weight or more and 5% by weight or less.

Next, we also investigated the amount of activated carbon added when using it as a conductive agent for the auxiliary network, but this case also has an effect on the capacity reduction rate, and the practically effective mixing amount is the same as that of granular graphite mentioned above. 2% by weight or more, 5% by weight
Met.

Example 4 As mentioned above, the formation of the auxiliary network was effective in reducing the capacity reduction rate, but in order to further enrich the network, the charge adsorption effect was used in the process of adding and mixing the conductive agent for the auxiliary network. A study was conducted to achieve more uniform mixing. Generally, it is said that the particle dispersibility of the powder becomes more uniform when the charge adsorption effect is used compared to simple mixing.

Therefore, a powder consisting of composite particles in which 5% by weight of AB is immobilized with respect to MnO2 and granular graphite powder with 5% by weight relative to MnO2 are stirred while being dispersed in the vapor phase, and the charged adsorption effect due to contact friction is applied to the composite particles. A mixture was prepared in which granular graphite powder was adsorbed on. A comparative study was then conducted using a battery similar to the one described above and a mixture of the same composition made by simply mixing granular graphite powder in a mixer.

Figure 6 is a capacity-cycle characteristic diagram of the two types of batteries mentioned above.
The graph is plotted up to 00 cycles, and shows the characteristics of a battery that uses a charge adsorption effect to add and mix granular graphite for the auxiliary network (indicated by a solid line 27), and the characteristics of a battery that simply uses a mixer (indicated by a broken line 28). It shows. From this figure, it can be seen that if the charge adsorption effect is used to add and mix the conductive agent for the auxiliary network, the reduction in the rate of capacity decrease due to cycles is further promoted.

A similar effect was also confirmed when activated carbon was used as a conductive agent for the auxiliary network.

As mentioned above, it was effective to use the charge adsorption effect to add and mix the conductive agent for the auxiliary network, and we believe that the use of the high-speed air impact method in addition to this can further enrich the auxiliary network. Thought.

Therefore, after treating the mixture prepared by the above-mentioned charge adsorption effect by a high-speed air impact method, it was constructed into a battery and studied.

However, in this case, the utilization rate became extremely poor as shown in curve 29 (product treated by impact method in high-speed air flow) in Figure 6.This is because granular graphite, which is generally said to have poor liquid absorption, is strong. This is thought to be because the conductive agent is fixed on the MnO2 surface and its wettability with the electrolyte is reduced.Therefore, as a method of mixing and adding the conductive agent for the auxiliary network, general mixing using a mixer, preferably charging adsorption, is preferable. The previous method is suitable.

Effects of the invention By using the positive electrode of the present invention and its manufacturing method,
It is possible to further suppress a decrease in charge/discharge capacity due to the positive electrode, and to provide a lithium secondary battery having an extremely stable discharge capacity regardless of the progress of the cycle.

[Brief explanation of drawings]

Figure 1 is a schematic diagram of composite particles according to the present invention, Figures 2 and 4.
5 and 6 are comparative diagrams of capacity cycle characteristics showing the effects of the present invention, and FIG. 3 is a longitudinal cross-sectional view of a battery used in an example. 1...Manganese dioxide particles, 2...Carbon material fine powder, 3...Positive electrode, 4...Positive electrode case, 5...Titanium Net, 6...
Separator, 7... Sealing plate, 8... Negative electrode, 9... Electrolyte, 10... Gasket, 11-29... Charge Discharge-cycle characteristic curve. Agent's name: Patent attorney Shigetaka Awano (1 person) 1st figure - - = Hs and man fshi rice stand) 2 - - carbon buozui mi rice beam layer ψ - L6 b > flash selection Makoto W- ICI-f Sumiri 2o-2tt% 2f-j Emperort% 22-4 tty 23-・-ytt! + 2φ-6 斐ty 2! ;-1 head tz 2cm--Nesoji 4 Kenmi-no-no-6 Fig. 271--fr4 knot iDhmi] 1 Ijidote] A giant nai L2
8--Myasu Ichiboku Introduction Product 29--Takahigashi 5 Trap Reception V Juice Carry Type Product Zaikuru Shio,

Claims (7)

[Claims] (1) A positive electrode whose main constituent materials are manganese dioxide as an active material and carbon material as a conductive agent, in which a part of the conductive agent has an average particle size of 10^-^1 to 10^-^5 with respect to the manganese dioxide particles. A positive electrode for a lithium secondary battery, characterized in that it is a carbon material fine powder having a diameter ratio, and has composite particles in which the carbon material fine powder 2 is immobilized on the surface of manganese dioxide particles 1. (2) The positive electrode for a lithium secondary battery according to claim 1, wherein the composite particle powder contains granular graphite powder or activated carbon powder as a conductive material other than the carbon material fine powder. (3) Claim 1, characterized in that the carbon material fine powder of the composite particles is carbon black, and the amount of the carbon material fixed on the surface is 2% by weight or more and 5% by weight or less based on manganese dioxide. The positive electrode for a lithium secondary battery described above. (4) The carbon material fine powder of the composite particles is fibrous graphite, and the amount of the carbon material fixed on the surface is 1% by weight or more and 3% by weight or less based on manganese dioxide. A positive electrode for a lithium secondary battery as described in . (5) The positive electrode for a lithium secondary battery according to claim 2, wherein the granular graphite powder or activated carbon powder is present in an amount of 2% by weight or more and 5% by weight or less based on manganese dioxide. (6) The composite particles described in claim 1 are stirred together with granular graphite powder or activated carbon powder while being dispersed in a gas phase, and the granular graphite powder or activated carbon powder is adsorbed onto the composite particles by the electrostatic adsorption effect caused by contact friction. A method for producing a positive electrode for a lithium secondary battery, characterized by: (7) Manganese dioxide powder and carbon material fine powder are stirred while being dispersed in a gas phase, and the carbon fine powder is adsorbed onto the manganese dioxide particles by the charge adsorption effect caused by contact friction. Claims characterized in that carbon material fine powder is implanted and fixed on the surface of manganese dioxide using a method in which the powder is introduced into an air stream having a rotating body rotating at high speed and the powder particles are strongly collided with each other. The method for producing a positive electrode for a lithium secondary battery according to item 6.