JP2000058039A - Evaluation method for battery constituting material, and battery material selected in the method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the performance of a lithium ion battery in short term by improving evaluation relying on experience in prior arts, when selecting and evaluating electrode material of the lithium ion battery and establishing a theoretically selecting method, in order to improve charging/discharging characteristic. SOLUTION: Surface free energy of a material constituting a lithium ion battery is measured, wettability value between both materials of active material and binder and an electrolyte is calculated from the values of the surface free energy, a contact angle between the electrolyte and the binder, and a contact angle between the electrolyte and the active material, and thereby optimization of combination of electrode materials affecting charging/discharging characteristic of lithium ion battery can be estimated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a field having a lithium ion secondary battery, a lithium battery and other battery structures, wherein a material called a binder is used. Here, the electrolyte solution consists of an electrolyte and a solvent, or an electrolyte and a polymer, or an electrolyte, a polymer, and a solvent. The electrode material consists of a material containing an active material and a binder.

[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 power supply for driving these devices. In this respect, non-aqueous secondary batteries, especially lithium ion secondary batteries, are expected to have high voltage and high energy density.

In particular, a battery system using a lithium composite oxide such as LiCoO2 or LiNiO2 as a positive electrode active material and a carbon material as a negative electrode material has recently attracted attention as a high energy density lithium ion secondary battery. The features of this battery system are that the battery voltage is high and that both the positive and negative electrodes use an intercalation reaction. A battery using LiCoO2 as a positive electrode and a carbon material as a negative electrode has already been commercialized. In such a lithium-ion secondary battery, it is an important element to perform a uniform charge / discharge reaction, and therefore, in most cases, both the positive electrode and the negative electrode contain a metal foil collector containing an active material. A sheet-shaped electrode plate coated with an agent layer is used. The material of the current collector is made of aluminum or aluminum for the current-collecting metal foil of the positive electrode because it is electrochemically stable at each operating potential when used in batteries.
Copper is used for the metal foil of the negative electrode. In the case of an electrode plate in which a mixture layer is formed by coating on such a metal foil, a mixture containing an active material, a conductive material, and a binder must be formed into a paste suitable for coating. Some paste techniques suitable for coating have been reported. For example,
In order to apply a mixture paste obtained by mixing and kneading LiNiO2 and a conductive agent in an organic solvent, a PVDF resin has conventionally been used as a binder. This PVDF resin is
The active material and the conductive agent particles exhibit binding properties in the form of PVDF resin agglomerates interposed between the active material and conductive agent particles. 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. On the other hand, for example, the PTFE resin exhibits binding properties in a form in which fibrous particles of the PTFE resin are entangled with the particles of the active material or the conductive agent in a network. It is also reported that PTFE resin does not easily swell even when it absorbs the electrolytic solution, so that it can maintain good adhesion to the active material and the current collector without expanding between the particles of the active material and the conductive agent. ing.

[0004] Many patents have been filed,
There are many patent applications for lithium-ion batteries using empirically inert binders. Especially the conventional PVD
A patent improved with reference to the use of F resin is disclosed in
-250127, JP-A-9-82314, JP-A-9-19-1
No. 99132, Japanese Patent Application Laid-Open No. 9-199133, and the like, and the performances of the binder are evaluated in Japanese Patent Application Laid-Open Nos. 8-106897, 8-172452, and 8-1996.
-130016, and Japanese Patent Application Laid-Open No. 5-182692 have been filed.

[0005]

However, it can be said that there is no application patent examined in principle regarding the method of selecting the binder or the method of evaluating the binder, and there is no other technical report. The reason is, for example, in the case of a positive electrode, there are many combinations of active materials, current collectors, and conductive agents. Until now, active materials, current collectors, and binders have been selected. This is because it involves the troublesome work of forming a battery structure, conducting a charge / discharge cycle test, again determining the amount of binder to be added from the crude data, and further improving the binder.
Therefore, it is preferable that the binder to be used can be determined by determining the active material, the current collector, the electrolyte, and the like of the positive electrode.

[0006]

[Means for Solving the Problems] The performance of a lithium ion secondary battery is finally determined by conducting a charge / discharge cycle test.
As can be seen from the data, at least the performance of a lithium-ion secondary battery involving a binder can be determined by forming a fine capillary tube between the active material and the binder, and allowing the electrolyte to penetrate there. If it is considered that this will affect the battery capacity in the discharge cycle, it is considered that the ease of penetration of the electrolyte can be expressed by evaluating the wettability (ease of wetting) of the capillary. The change in free energy before and after infiltration of the electrolyte is a measure of wettability, but the surface free energy of two different or identical materials, such as the gap between the active material and the binder, is different or the same. The change in surface free energy before and after the infiltration of the electrolyte in the case of is measured by measuring the surface tension, contact angle, etc., and calculating the numerical value from the obtained value. In comparison, different active materials,
It is possible to estimate the battery discharge capacity of a lithium-ion battery composed of a binder and an electrolyte.

[0007] represents the intrinsic adhesion, adhesion work Wa between the substance A and the substance B ^{_{is, Wa = 2 (γ A d}} * γ B d) 1/2 or, 2 (gamma _{A
^{d * γ B d) 1/2 +2}} (γ A p * γ B p) 1/2 ^{_{or, 2 (γ A d * γ}} B d) 1/2 +2 (γ A p *, γ B
_{^{p) 1/2 p +2 (γ A}} h * γ B h) 1/2 ^{_{or, 2 (γ A d * γ}} B d) here ^1/2 + _{W AB,} ^γ _i ^d, the surface free energy of substance i Γ _i ^p represents the dipole force of the surface free energy of the substance i, and γ _i ^h represents the hydrogen bonding force of the surface free energy of the substance i. , Japan Society of materials Science, ed., Mohanabo) in _{addition, W AB} represents the work of adhesion by the acid-base reaction of Fowkes proposed. On the other hand, the change in the free energy at the interface between the substance A and the substance B due to the intrusion of the solvent (S) such as the electrolytic solution (G) can be expressed as G = γ (COS θ _A + COS θ _B ). Here, COS .theta _A represents the contact angle between the solvent S of substances A, COS .theta _B represents the contact angle between the solvent S of the material B. (References: Colloids and Surfac
Adhesion in es, vol 9, (1984)
Model and Experimental Ver
informationFor Polymer-SiO2
System. If the value of ▲ G is a relatively large value, the solvent (electrolyte) easily permeates the interface, and if the value is relatively small, it is difficult for the solvent to permeate the interface. ing.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Next, the present invention will be described in more detail with reference to preferred embodiments. The electrolyte for a non-aqueous electrolyte secondary battery of the present invention is characterized by a method of forming a positive electrode material on a current collecting material and a positive electrode material selected by the method.

The current collector used in the present invention includes, for example, metal foils such as aluminum and copper. The thickness of the metal foil is about 10 to 30 μm.

The dry thickness of the positive electrode layer applied to the metal foil current collector of the present invention is preferably in the range of 0.001 to 5 μm.

The positive electrode active material used in the present invention includes:
For example, LiCoO2, LiNiO2, LiMn2O4
Such as lithium oxide, TiO2, MnO2, MoO3
One or a plurality of chalcogen compounds such as V2O5 are used in combination. On the other hand, as the negative electrode active material, metal materials such as lithium metal, lithium alloy, graphite, carbon black, and acetylene black, or materials intercalating lithium ions are preferably used. In particular, by using LiCoO2 as a positive electrode active material and a carbonaceous material as a negative electrode material, a lithium secondary battery having a high discharge voltage of about 4 V can be obtained.

In particular, in the present invention, when an aluminum foil is used as the current collector and a lithium metal oxide is used as the active material, excellent bonding ability to both the aluminum foil and the lithium metal oxide is obtained. As a result, the adhesion of the positive electrode active material layer to the aluminum foil current collector is remarkably excellent, and the active material layer may peel, fall off, or crack during the battery assembly process and during charging and discharging. It is possible to provide an electrode plate for non-aqueous electrolyte secondary batteries. In addition, the electrolyte has excellent wettability to the surface of the active material, and therefore an electrode plate for a non-aqueous electrolyte secondary battery having excellent charge / discharge performance can be provided.

It is preferable that these active materials are uniformly dispersed in the formed coating layer. The binder used in the preparation of the coating solution containing the active material may be a polyolefin resin, a cyclic diene polymer, a cellulose fiber, a polybenzcyclobutene, a polyphenylene sulfide, a fluorinated polymer, or the like selected by the method of the present invention. A copolymer of a polymer and another polymer or a mixture of the polymer and another polymer can be used. However, the selected polymer is not limited to the above-mentioned polymer.

A specific method for preparing a coating solution containing an active material used in the present invention will be described. First, a particulate binder and a powdered active material appropriately selected from the above-mentioned materials are put into a dispersion medium composed of a solvent such as NMP, water, and the like. The mixed composition is prepared by mixing and dispersing using a conventionally known dispersing machine such as a homogenizer, a ball mill, a sand mill, and a roll mill. At this time, the mixing ratio of the binder and the active material may be the same as that conventionally used.

The active material coating solution is applied on the surface of the metal foil current collector in a dry thickness of 10-200 by using various coating methods.
After coating in the range of microns, preferably 50-180 microns, heat dry.

Further, in order to further improve the homogeneity of the coating layer formed by the coating and drying processes as described above, the coating layer is subjected to a press treatment using a metal roll, a heating roll, a sheet press or the like. To form the electrode plate of the present invention. Further, before the process of assembling the battery using the above-described electrode plate, it is preferable to further perform a heat treatment, a pressure reduction treatment, or the like in order to remove moisture in the active material layer of the electrode plate.

When a positive electrode non-aqueous electrolyte secondary battery electrode plate of the present invention prepared as described above is used, for example, when a lithium secondary battery is manufactured, the solute lithium is used as the electrolyte. A non-aqueous electrolyte in which a salt is dissolved in an organic solvent is used. Examples of the solute lithium salt forming the non-aqueous electrolyte include LiCLO4, LiBF4, and LiPF.
Inorganic salts such as 6, LiAsF6, LiCL, LiBr, and LiB (C6H5) 4, LiN (SO2CF
3) 2, LiC (SO2CF3) 3, LiOSO2CF
3, LiOSO2C2F5, LiOSO2C3F7, L
iOSO2C4F9, LiOSO2C5F11, LiO
Organic titanium salts such as SO2C6F13 and LiOSO2C7F15 are used.

Examples of the organic solvent used at this time include cyclic esters, chain esters, cyclic ethers, and chain ethers. As cyclic esters, for example, propylene carbonate, butylene carbonate. Gamma-butyrolactone, vinylene carbonate, 2
-Methyl-gamma-butyrolactone, acetyl-gamma-butyrolactone, gamma-valerolactone and the like.

Examples of the chain ester include dimethyl carbonate, diethyl carbonate, dibutyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl butyl carbonate, methyl propyl carbonate, ethyl butyl carbonate, ethyl propyl carbonate, butyl propyl carbonate, and propion. And alkyl esters of malonic acid, alkyl esters of acetic acid, and the like.

Examples of the cyclic ethers include tetrahydrofuran, alkyltetrahydrofuran, dialkylalkyltetrahydrofuran, alkoxytetrahydrofuran, dialkoxytetrahydrofuran,
Examples of chain-ters include 1,3-dioxolan, alkyl-1,3-dioxolan, 1,4-dioxolan and the like, for example, 1,2-dimethoxyethane, 1,1
Examples thereof include 2-diethoxyethane, diethyl ether, ethylene glycol dialkyl ether, diethylene glycol dialkyl ether, triethylene glycol dialkyl ether, and tetraethylene glycol dialkyl ether.

[0021]

【Example】

Example 1) The constituent materials of the battery structure were first determined as follows. LiCoO2, conductive agent; acetylene black, graphite binder; PE, PVDF, negative electrode; lithium metal separator; polyolefin-based separator electrolyte; EC (ethylene carbonate) / DMC (dimethyl carbonate); 1 L (ratio) ; 1/2) and 1 mol of electrolyte LiPF6 (lithium hexafluorophosphate). Current collector; positive electrode: aluminum; negative electrode: stainless steel

Example 2) Next, the contact angles of the surfaces of various materials were measured. An active material LiCoO2 was formed on a glass substrate by a sputtering method, and used as a positive electrode for measurement. The surface contact angle of LiCoO2 with an electrolyte, alpha bromonaphthalene, n-dodecane, methylene iodide, and water was measured at a predetermined temperature (18 degrees and 22 degrees). Degrees), the contact angles and the surface free energy calculated from those contact angles were calculated to obtain numerical values. Similarly, the binders PE and P
VDF values were obtained. The results are shown in (Table 1). Here, in order to give a relative comparison of the values of the surface free energy calculated from the contact angle,
C (ethylene carbonate) / DMC (dimethyl carbonate); 1 L (ratio; 1/2)
The mole was used as the compounding solution.

From the obtained values, a comparison value (Wb) between the original adhesion (Wa) and the change in surface free energy (G) accompanied by the phenomenon of infiltration of the electrolyte was examined. The results are shown in (Table 2).

[Table 1]

[Table 2]

Example 3) A coin cell of a lithium ion battery was formed using the same constituent materials as in Example 1), and a charge / discharge cycle test was performed. Constituent materials; Positive electrode; powdered LiCoO2 (manufactured by Honjo Chemical Co., Ltd.) as active material, acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd.) and graphite (SP-300H manufactured by Nippon Graphite Co., Ltd.) as conductive agents, and PVDF (Daikin as binder) Using PE (manufactured by Sumitomo Seika). Negative electrode; metallic lithium; separator; polyolefin-based separator, electrolytic solution; LiPF6 / EC / DM same as in Example 1) above
C (1 mol / 1 L (1/2)) positive electrode current collector; aluminum; negative electrode current collector; stainless steel;

Charge / discharge test conditions;

First, the active material, acetylene black and graphite were mixed with a binder.

When the binder is polyethylene (PE), the binder is dispersed in water and applied to the aluminum of the current collector.
Dried with a 10-ton press and a diameter of 15
The plate was cut out into a disk having a diameter of 1 mm and pressed again with a 1-ton press to obtain a positive electrode.

When the binder is PVDF, N-
Dissolved in methylpyrrolidone (NMP), applied to the aluminum of the current collector, dried at 160 ° C., pressed with a 10 ton press, cut into a 15 mm diameter disc, and pressed again with a 1 ton press This was used as the positive electrode.

A lithium ion secondary battery was constructed using the above-mentioned constituent materials, and was charged at a constant current of 0.4 mA / cm ^{2 with} respect to the positive electrode at room temperature (20 degrees Celsius). 0.3 volts and the discharge end voltage was 3.7 volts.

The discharge capacity in each cycle was compared.

[Table 3]

As described above, according to the present invention, it is possible to provide an electrode plate for a non-aqueous electrolyte secondary battery in which an electrolyte has excellent wettability with respect to a positive electrode active material and a binder. .

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Claims (15)

[Claims] 1. A method for evaluating battery constituent materials by calculating surface free energies of a binder, an active material, and an electrolyte. 2. A binder having a relatively large value of the following formula:
Lithium ion battery using active material and electrolyte solution Surface free energy of electrolyte solution; γ Contact angle of electrolyte solution with binder; θ _B Contact angle of electrolyte solution with active material; θ _M γ (COS θ _P + COS θ _Q Here, θ _P and θ _Q are either θ _S or θ _M. 3. The lithium ion battery according to claim 2, wherein the binder for the electrode is a polyolefin and another polymer. 4. The lithium ion battery according to claim 2, wherein the binder for the electrode is a diene polymer and another polymer. 5. The lithium ion battery according to claim 2, wherein the binder of the electrode is a cellulose fiber and another polymer. 6. The lithium ion battery according to claim 2, wherein the binder for the electrode is polybenzcyclobutene and another polymer. 7. The lithium ion battery according to claim 2, wherein the binder for the electrode is polyphenylene sulfide. 8. The lithium ion battery according to claim 2, wherein the binder of the electrode is a fluoropolymer and another polymer. 9. The lithium ion battery according to claim 2, wherein the binder for the electrode is a polyimide polymer and another polymer. 10. The lithium ion battery according to claim 3, wherein the polyolefin is polyethylene. 11. The lithium ion battery according to claim 3, wherein the polyolefin is polypropylene. 12. The lithium ion battery according to claim 4, wherein the diene polymer is a cyclized isoprene polymer. 13. The lithium ion battery according to claim 4, wherein the diene polymer is a cyclized butadiene polymer. 14. The lithium ion battery according to claim 4, wherein the other polymer is a polyolefin. 15. The electrolyte according to claim 2, wherein the electrolyte is a polymer electrolyte.
Lithium-ion battery