JP3067544B2 - Positive electrode for lithium secondary battery and method for producing the same - 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, and more particularly to an improvement in the characteristics of its positive electrode.

[0002]

2. Description of the Related Art In recent years, portable and cordless electronic devices have been rapidly advancing, and there is a high demand for a small and lightweight secondary battery having a high energy density as a drive power source for these devices. In this respect, non-aqueous secondary batteries, especially lithium secondary batteries, are expected to have high voltage and high energy density.

In particular, a battery system using a lithium composite oxide such as LiCoO ₂ or LiNiO ₂ as a positive electrode active material and a carbon material as a negative electrode active material has recently attracted attention as a high energy density lithium secondary battery. . The features of this battery system are that the battery voltage is high and that the positive and negative electrodes use an intercalation reaction. Since no metallic Li is used for the negative electrode, there is no short circuit due to the precipitation of dendritic Li and it is safe. And quick charging. A battery using LiCoO ₂ as a positive electrode and a carbon material as a negative electrode has already been commercialized. In the case of such a lithium secondary battery, it is an important element to perform a uniform charge / discharge reaction.In many cases, both the positive electrode and the negative electrode are coated with a mixture layer containing an active material on a metal foil current collector. A sheet-shaped electrode plate is used. In addition, the material of the current collector is electrochemically stable at each operating potential when used in a battery, so aluminum (Al) is used for the current collector metal foil for the positive electrode and metal foil for the negative electrode is used for the current collector. Is made of copper (Cu) or the like. In the case of an electrode plate that creates a mixture layer by coating on such a metal foil,
It is necessary to make a mixture containing an active material, a conductive agent and a binder into a paste suitable for coating, and some pasting techniques suitable for coating have been reported. For example, in a paste using an organic solvent, a carbon powder of a conductive agent is mixed with an active material, and then the mixture is kneaded with a liquid in which PVDF as a binder is previously dissolved in an organic solvent such as NMP to form a paste. There are methods. On the other hand, in the aqueous paste, after mixing carbon powder of a conductive agent with an active material, this mixture is previously mixed with carboxymethyl cellulose (CM) as a thickener.
There is a method of kneading with an aqueous solution in which C) or the like is dissolved, and then adding an aqueous dispersion of PTFE and kneading to form a paste. When LiCoO ₂ is used as an active material,
A relatively good electrode plate can be formed by applying an organic solvent-based paste or an aqueous solution-based paste, and in any case, it is used in a commercialized battery. However, LiN
When iO ₂ was used as an active material, when an aqueous paste was prepared, the active material was deteriorated by moisture. In the case of using LiNiO ₂ as described above, the use of an aqueous solution-based paste significantly deteriorates battery characteristics. Therefore, it is necessary to use an organic solvent-based paste. Further, a composite oxide such as LiNi _x Co _1-x O _{2 in} which a part of Ni of LiNiO ₂ is replaced by an element such as Co or Mn has been proposed as a promising active material.
Since it is deteriorated by moisture similarly to iO _2, it is necessary to use an organic solvent-based paste.

In order to apply a mixture paste obtained by mixing LiNiO ₂ and a conductive agent in an organic solvent and kneading the mixture, a PVDF resin has conventionally been added as a binder. This P
As shown in FIG. 1 (A), the VDF resin exhibits a binding property in the form of a mass of PVDF resin particles 2 interposed between particles 1 of an active material or a conductive agent, but swells when absorbing an electrolytic solution. Since the degree is large, the distance between the particles of the active material and the conductive agent is increased, and as a result, the adhesion between the active material and the conductive agent and the current collector is reduced, and the current collection efficiency is reduced.

[0005] On the other hand, PTFE resin is insoluble in water, but is dispersed in water using a CMC solution.
Was added to the active material and the conductive agent and kneaded to prepare a paste.

As shown in FIG. 1 (B), the PTFE resin exhibits binding properties in a form in which fibrous particles 3 of the PTFE resin are intertwined with particles 1 of an active material or a conductive agent in a network. In addition, since the PTFE resin does not swell even when absorbing the electrolytic solution, it does not spread between the particles of the active material and the conductive agent, and can maintain good adhesion between the active material and the conductive agent and the current collector. it can.

[0007]

However, LiN
When iO ₂ is used as an active material, an organic solvent must be used instead of water as a solvent.
Dispersing E has not been realized so far.

Therefore, with an organic solvent in which LiNiO ₂ and a conductive agent are mixed, a paste having excellent current collecting properties and binding properties cannot be obtained by making use of the properties of a PTFE resin which does not swell with an electrolytic solution. .

The present invention solves such a problem, and uses LiNiO ₂ as a positive electrode active material,
After mixing iO ₂ and a conductive agent in an organic solvent, further adding a binder and kneading to obtain a paste of a positive electrode mixture, when applying this paste to a metal foil, the current collecting property and the binding property It is an object of the present invention to provide an excellent paste.

[0010]

In order to solve the above-mentioned problems, the present invention provides a positive electrode mixture layer comprising a positive electrode active material powder, a conductive agent and a resin binder on a metal foil of a current collector core material. For the positive electrode formed by coating, the binder is a polytetrafluoroethylene (PTFE) resin as a first binder, a polyvinylidene fluoride (PVDF) resin, a polyvinylidene chloride (PVD) resin as a second binder. PVDC) containing at least one resin selected from the group consisting of: Preferably, the content of the first binder is 2% by weight or more and 8% by weight or less based on the weight of the active material, and the content of the second binder is 2% by weight or less based on the weight of the active material. % By weight and 6% by weight or less, and the sum of the contents of the first and second binders is 10% by weight or less based on the weight of the active material.

[0011]

According to the present invention, in order to disperse a PTFE resin in an organic solvent, a PVDF resin or a PVDC resin obtained by replacing fluorine of PVDF with chlorine is added to the organic solvent.

As a result, the PTFE resin is uniformly distributed in the positive electrode mixture, and a mixture paste having appropriate viscosity and fluidity can be obtained. Further, the PVDF resin or the PVDC resin has an effect of improving the binding property between the metal foil and the paste.

For this reason, PVDF is used for PTFE resin.
If the amount of the resin or PVDC resin is too small, the mixture layer tends to peel off from the metal foil.

On the other hand, if the amount of the PVDF resin or the PVDC resin is too large relative to the PTFE resin, the aggregated particles of the PVDF resin or the PVDC resin are interposed between the active material and the conductive agent particles, so that the aggregated particles become resistant. Acting as a component, the polarization characteristics of the electrode plate are reduced.

Further, considering that the organic solvent used for preparing the paste is such that the PVDF resin or the PVDC resin is dissolved to have an appropriate viscosity and that the boiling point is at an appropriate temperature, N-methyl-2- Pyrrolidone (NMP) is best.

[0016]

Embodiments of the present invention will be described below with reference to the drawings.

(Example 1) LiCoO as a positive electrode active material
₂ , using acetylene black (AB) as a conductive agent
5% by weight based on the weight of the active material to prepare a mixture,
This was used to study the formation of a paste. First, PVD
Production of an electrode plate was attempted using only the F resin as a binder. NMP
The mixture was kneaded using a solution prepared by dissolving PVDF in an amount of 15% by weight with respect to the weight of the active material so that the binder had a predetermined value to prepare a paste. Note that NMP was used as a diluent for adjusting the viscosity of the paste. Next, a paste was applied to one side of the Al foil using a doctor blade method, dried with hot air at 80 ° C., and then rolled with a roller press. As a result of trial production of electrode plates having various PVDF contents, when the PVDF content was less than 2% by weight with respect to the active material, the mixture layer after drying was easily peeled off. The layer has fallen off. When only PVDF is used, at least the PVDF content is desirably 3% by weight or more based on the active material. As a result, PV
The DF content is 3, 4, 5, 6, 7, 8,
Prototypes of 9, 10% by weight of electrode plates were produced.

Next, an electrode plate was prepared using an aqueous PTFE dispersion and using only the PTFE resin as a binder. The same mixture as described above was previously kneaded with a 1% by weight CMC aqueous solution to prepare a paste, to which a PTFE dispersion was added and further kneaded to obtain a paste for coating.
In this case, if the PTFE dispersion is added first and kneaded, the PTFE fibers become entangled.
Even if the mixture is kneaded with the C aqueous solution, a fluid paste cannot be obtained again. For this reason, it is important to use a mixture kneaded with an aqueous CMC solution. In this case, similarly, a paste was applied to one surface of the Al foil using the doctor blade method, dried with hot air at 80 ° C., and then rolled with a roller press. As a result, the PTFE content was 2% based on the active material.
If the value is less than 10%, the mixture layer has fallen off the Al foil,
The content of E is desirably 3% by weight or more with respect to the active material, and the content of PTFE is 3, 4,
5, 6, 7, 8, 9, and 10% by weight of electrode plates were prototyped.

FIG. 2 is a longitudinal section of a coin-type battery used in the embodiment. In FIG. 2, a positive electrode 1 is obtained by punching out the above-mentioned electrode plate obtained by drying and rolling an electrode plate formed by coating one side of an Al foil on a disk, and is provided inside a positive electrode case 2. . The counter electrode 3 is formed by pressing metallic lithium on a stainless steel net 5 fixed to the inside of the sealing plate 4 by spot welding. These were sealed together with a polypropylene separator 6 and an electrolytic solution 7 via a polypropylene gasket 8 to obtain a completed battery having a diameter of 20 mm and a height of 1.6 mm. The electrolyte used was one in which 1 mol of lithium hexafluorophosphate (LiPF ₆ ) was dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC).

The normal charge / discharge of this test battery is performed at room temperature (2
(0 ° C.) at a constant current of 0.5 mA / cm ² with respect to the positive electrode, at a charge termination voltage of 4.3 V and a discharge termination voltage of 3.0 V. In addition, only the discharge in the fifth cycle during the normal charge / discharge was performed at a current of ^2.5 mA / cm ^{2 in} order to confirm high-rate discharge characteristics.

Further, the evaluation of the storage characteristics in a high-temperature environment was carried out in such a manner that after the completion of the 10th charge / discharge cycle, the battery was taken out, stored in a 60 ° C. environment for 20 days, and then charged and discharged at room temperature again. This is to compare the charge and discharge characteristics before and after storage.

First, a high-rate discharge performance is obtained. A test battery experimentally manufactured using an electrode plate using PVDF or PTFE having various contents described above is charged and discharged under the above conditions, and the discharge capacity at the fourth cycle (0. ratio of 5 mA / cm ² discharge) for the 5th cycle discharge capacity (2.5 mA / cm ² discharge) (%), i.e. were compared utilization rate discharge. The results are shown in (Table 1).

[0023]

[Table 1]

In the ordinary discharge (0.5 mA / cm ² discharge), the capacity was about 140 mAh / g per weight of the active material regardless of the binder content.

As is clear from Table 1, when the binder content increases, the high-rate discharge utilization rate tends to decrease. In the case of PVDF, the utilization rate decreases when the content is 7% or more. Becomes significant. On the other hand, in the case of PTFE, the decrease in the utilization rate relative to the content is smaller than that of PVDF, and the content is 1%.
It was relatively good even at 0%. Therefore, PTFE,
If an electrode plate as described above is produced using a binder using PVDF alone, the content of PVDF is 6%.
% Or less, and in the case of PTFE, an increase in the content results in a reduction in the amount of the active material filled in the electrode plate.

Next, the results of the high-temperature storage characteristics will be described. FIG. 3 shows the change in the discharge capacity with the cycle in the case of PVDF.
The characteristics after the 10th cycle are those after storage. The capacity at the fifth cycle is low, which is the result of the high-rate discharge test described above. In the case of PVDF, the capacity decrease accompanying the cycle after storage was extremely large, regardless of the binder content.

Similarly, FIG. 4 shows the result in the case of PTFE, but there is almost no decrease in capacity with the cycle after storage regardless of the binder content. From the above results, PTFE is an effective binder in terms of storage characteristics.

(Example 2) Next, LiNiO ₂ was used as an active material.
A study was conducted when using the. PVDF as above,
In addition, an electrode plate was prototyped using PTFE.

In the case of LiNiO ₂ , PVD is used as a binder.
As a result of applying NMP paste to which F
Almost the same electrode plate as in the case of O ₂ was obtained, and the strength was sufficient when the binder content was 3% by weight or more.
On the other hand, in the case of the aqueous paste using PTFE, the paste foamed after being applied on the Al foil. As a result of the analysis, the foam was hydrogen, and the cause was that the pH of the paste was high (alkaline) and the aluminum foil of the core material was corroded. Although the strength of the electrode plate after drying was considerably reduced due to foaming, rolling was performed, and the battery was charged and discharged in the above-described test battery. However, the result was a remarkably small capacity (30 mAh / g or less per active material weight). After drying LiNiO ₂ once washed with water, PVD
Even when applied with an organic solvent-based paste using F, the capacity is extremely small (the capacity is not reduced even if the same treatment is performed with LiCoO ₂ ), so that LiNiO ₂ itself comes into contact with water. It is presumed to have deteriorated. As described above, in the case of LiNiO ₂ , at least an organic solvent-based paste must be used as long as a coated electrode plate is manufactured.

FIG. 5 shows the pace with NMP to which PVDF was added.
LiNiO when coated and coated _TwoSame as above for positive electrode
Results of charge / discharge cycle tests including high-rate discharge and storage
The result is shown. FIG. 5 shows the PVDF content in the same manner as above.
The characteristics in the case of 3 to 10% are described. Normal discharge
(0.5mA / cm^TwoDischarge) regardless of the binder content
The capacity is about 180 mA per active material weight.
h / g. However, also in this case, LiCoO_TwoSame as
Results in significant cycle deterioration after storage
Was.

Similar investigations using PVDC instead of PVDF showed that the high-rate discharge performance was almost the same as that of PVDF, but the storage characteristics were PVD.
Performance was lower than DF.

[0032] The fact that PTFE can be a binder having excellent storage characteristics is due to the solvent resistance and morphology of PTFE that are unique to PTFE.

Therefore, an attempt was made to prepare an organic solvent-based paste using PTFE as a binder. Organic solvents include methyl ethyl ketone (MEK), toluene (TR), NMP
For example, NMP was used because it was the easiest solvent to use as far as we know from the viewpoints of cost, effects on the human body, odor and boiling point.

The PTFE used was a solid material which was also a raw material of the aqueous dispersion, but was in an agglomerated state (granules in which fibers were already entangled in large lumps), and NMP was used.
It was put into the container and stirred, but it was impossible to disperse it uniformly. Thus, several nonionic surfactants were simultaneously added to prepare an organic solvent-based dispersion, but it was not possible to regenerate the fiber once entangled.

Then, PTFE fibers having a relatively small entanglement were obtained, and PTFE in the form of a fine powder was obtained. Similarly, the PTFE was charged into NMP and stirred. However, coagulation occurred together with the stirring. So that no dispersion could be obtained. Also in this case, some surfactants were tried, but none of them was particularly effective. Therefore, PTFE is added to a viscous solution obtained by adding the following resin to NMP without going through dispersion.
An attempt was made to disperse the fine powder directly.

Polyamide (PA), polyimide (PI), polyvinylidene chloride (PVDC), polyvinylidene fluoride (PVD)
F) proved promising.

These were added to a viscous solution prepared by dissolving them in NMP.
As a result of charging and stirring the FE fine powder, a uniform dispersion state was obtained in each case.

Next, PTFE was dispersed in a viscous solution in which each of the above resins was dissolved in NMP, and a mixture in which a LiNiO ₂ active material and AB as a conductive agent were previously mixed was charged and stirred. As a result, pastes suitable for coating with fluidity were obtained. These pastes were applied to an aluminum foil, dried, and rolled to form an electrode plate.

Therefore, the battery was assembled in a test battery in the same manner as in the above-described example, and high-rate discharge and storage characteristics were evaluated. As a result, the initial capacity was good in each case. However, in the case of PA and PI, the high-rate discharge characteristics were extremely poor. Examination of the cause revealed that PA and PI resins were film-forming types and formed a film on the surface of the active material and conductive agent particles, and thus inhibited the charge / discharge reaction. The storage characteristics of the battery were much better as compared to the case where only the conventional PVDF and PVDC were used as the binder.

As described above, by adding PVDF or PVDC to PTFE, even when LiNiO ₂ is used as an active material and an organic solvent is used, an electrode plate having excellent current collecting properties and binding properties can be obtained using PTFE. I got it.

Example 3 PTFE as the first binder
And the results of a more detailed study on the binder content when PVDF is used as the second binder will be described.

The method for producing the paste was the same as in the above (Example 2), and PTFE was added to a solution in which PVDF was previously dissolved in NMP.
The fine powder is dispersed, and a mixture containing the same LiNiO ₂ active material as described above is added and kneaded. The content of each binder is changed by adjusting the amounts of PVDF and PTFE.

The content of PTFE as the first binder is in the range of 1 to 10% by weight with respect to the active material.
In the E content, the PVDF amount of the second binder is 1 to 10
Various pastes having different weight percentages were prepared, coated, dried, and rolled to produce a prototype of the electrode plate. As a result, it was found that if the PVDF was not contained at least at 2% by weight or more, sufficient adhesive strength to the Al core material was not obtained, and the mixture layer fell off in the rolling step. Also P
When the ratio of the PTFE content was higher than the VDF content, a phenomenon occurred in which the mixture layer was peeled from the aluminum foil into a film. For example, when the PVDF content was 2% by weight, the peeling phenomenon occurred when the PTFE content exceeded 8% by weight. If PVDF is 3% by weight or more, PTF
When E was 10% by weight or less, no peeling phenomenon was observed. Therefore, as shown in (Table 2) (in Table 2, ○ indicates a region where electrode plates can be formed, and x indicates a region where electrode plates cannot be formed), when PVDF is 2% by weight, PTFE is 8% by weight or less. In the range and the range where PVDF is 3% by weight or more, the electrode plate can be manufactured in the range where PTFE is 10% by weight or less.

[0044]

[Table 2]

Next, a test battery similar to the above was prepared using each electrode plate, and a charge / discharge cycle test including the same high-rate discharge and storage as described above was performed. In ordinary discharge, the capacity was about 180 mAh / g per active material weight in any case regardless of the content of the binder.

First, the high-rate discharge characteristic is that the PVDF content is 7%.
When the content was more than the weight%, the content was significantly reduced. In addition, even when the PVDF content was 6% or less, the high-rate discharge characteristics were greatly reduced.

Table 3 shows the results of the high-rate discharge utilization rate at various binder contents.

[0048]

[Table 3]

In Tables 3 and 4, the ○, ×, △, and-indications together with the values of the utilization rates indicate that ○ indicates that the utilization rate is 90% or more, Δ indicates that the utilization rate is less than 90%, and 80% or more. Things, ×
Means less than 80%, and-means that no electrode can be prepared.
As is evident from Table 3, the PTFE content is preferably at most 8% by weight and the PVDF content is at most 6% by weight. Furthermore, even in this range, the sum of the PTFE content and the PVDF content is at most 1
It is preferably 0% by weight or less. It can be seen that when the ratio exceeds this range, the utilization rate decreases significantly.

Next, regarding the high-temperature storage characteristics, FIG.
The results of the cycle characteristics before and after high-temperature storage of electrode plates having different PTFE contents when the VDF content is 3% by weight are shown. As shown in FIG. 6, the case where the content of PTFE is 1% by weight shows remarkable cycle deterioration after storage.
Even in the test in which the F content is different, those having a PTFE content of 1% by weight are not affected by the PVDF content.
In each case, remarkable cycle deterioration as shown in FIG. 6 was exhibited. This indicates that 1% by weight of PTFE is not enough to suppress the swelling of the electrode plate during storage. However, when the PTFE content is 2% by weight or more, P
Regardless of the VDF content, the cycle deterioration was good in any case as shown in FIG.

From the above results, in order to suppress cycle deterioration during high-temperature storage, the PTFE content should be at least 2%.
It is preferred that the content be at least 10% by weight. In addition, although the case where PVDC was used instead of PVDF was examined, the tendency regarding the binder content in the high-temperature storage characteristics was almost the same.

[0052]

As described above, the positive electrode of the present invention using LiNiO ₂ as an active material contains polytetrafluoroethylene (PTFE) resin as the first binder in the positive electrode mixture layer, As the second binder, polyvinylidene fluoride (PVD)
F) Resin, characterized by containing at least one kind of binder selected from the group of polyvinylidene chloride (PVDC), more preferably the content of the first binder in the mixture is an active material 2% by weight or more and 8% by weight or less based on the weight, and the content of the second binder is 2% by weight or more and 6% by weight or less based on the weight of the active material; The total of the binder content is at most 10% by weight based on the weight of the active material.
The following is assumed. As a result, a mixture paste having excellent current collecting properties and binding properties can be obtained, and high-rate discharge characteristics and storage characteristics of the battery can be improved.

[Brief description of the drawings]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A is a diagram showing a state of PVDF resin particles in a mixture. FIG. 1B is a diagram showing a state of PTFE resin particles in a mixture.

FIG. 2 is a sectional view of a coin-type battery.

FIG. 3 is a diagram showing a change in capacity with a cycle when a PVDF resin is added as a binder to LiCoO ₂ .

FIG. 4 is a diagram showing a change in capacity with a cycle when PTFE resin is added as a binder to LiCoO ₂ .

FIG. 5 is a diagram showing a change in capacity with a cycle when a PVDF resin is added as a binder to LiNiO ₂ .

FIG. 6 shows PTFE resin and P as a binder in LiNiO ₂ .
The figure which shows the capacity change with the cycle when adding VDF resin

[Explanation of symbols]

 1 Active material or conductive agent particles 2 PVDF resin particles 3 PTFE resin particles

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-7-153647 (JP, A) JP-A-6-52860 (JP, A) JP-A-59-173974 (JP, A) JP-A-4- 112459 (JP, A) JP-A-6-349482 (JP, A) JP-A-6-325573 (JP, A) JP-A-6-52889 (JP, A) JP-A-7-130357 (JP, A) (58) Field surveyed (Int.Cl. ⁷ , DB name) H01M 4/02 H01M 4/04 H01M 4/58 H01M 4/62 H01M 10/40

Claims (4)

(57) [Claims] 1. A positive electrode in which a positive electrode mixture layer comprising a positive electrode active material powder, a conductive agent and a binder of a resin is formed on a metal foil of a current collector core, wherein the binder is a first binder. Lithium containing at least one resin selected from the group consisting of a polytetrafluoroethylene (PTFE) resin as a binder and a polyvinylidene fluoride (PVDF) resin and a polyvinylidene chloride (PVDC) resin as a second binder A positive electrode for a secondary battery , wherein the content of the first binder is based on the weight of the positive electrode active material.
From 2% by weight to 8% by weight, and the content of the second binder is relative to the weight of the positive electrode active material.
And the sum of the contents of the first binder and the second binder is positive.
Lithium di-oxide not more than 10% by weight based on the weight of the polar active material
Positive electrode for secondary battery. 2. The method according to claim 1, wherein the positive electrode active material is lithium nickelate (Li).
_{2. The} positive electrode for a lithium secondary battery according to claim 1, wherein the positive electrode is a composite oxide obtained by substituting a part of Ni of LiNiO ₂ ) with another transition metal such as cobalt (Co) or manganese (Mn). 3. 3. A method for producing a positive electrode, wherein a positive electrode mixture layer comprising a positive electrode active material powder, a conductive agent and a binder of a resin is formed on a metal foil of a current collector core material. To prepare a solution in which the binder is dissolved,
Then, a positive electrode active material powder, a carbon powder, and a PTFE powder as a first binder are added to the solution, and these are added and kneaded to prepare a paste. Then, the paste is applied to a metal film. Of producing a positive electrode for a lithium secondary battery. 4. The method for producing a positive electrode for a lithium secondary battery according to claim 3 , wherein the organic solvent is N-methyl-2-pyrrolidone (NMP).