CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No. PCT/JP2016/077524, filed on Sep. 16, 2016, which claims priority from Japanese Patent Application No. 2015-184530, filed on Sep. 17, 2015, and Japanese Patent Application No. 2015-219948, filed on Nov. 9, 2015.

TECHNICAL FIELD

The present invention relates to a binder composition for a secondary battery, an electrode mixture paste for a secondary battery, a secondary battery using them, and a manufacture method thereof.

BACKGROUND ART

Lithium ion secondary batteries, which feature small size and large capacity, have been widely used as power supplies for electronic devices such as mobile phones and notebook computers and have contributed to enhancing convenience of mobile IT devices. In recent years, larger-scale applications, such as power supplies for driving motorcycles and automobiles and storage cells for smart grids, have attracted attention. As demand for lithium ion secondary batteries are increased and batteries are used in more various fields, characteristics such as a further enhancement in the energy density, lifetime characteristics for endurance over long-term use, and usability in a wide range of temperature condition are demanded.

Carbon materials are generally used in negative electrodes of the lithium ion secondary batteries, but it is examined to use silicon-based materials having large capability of absorbing and releasing lithium ions in the negative electrodes to enhance the energy density of the battery. However, there is a problem in that the silicon-based materials expand and contract by repeating charge and discharge of the lithium, and thereby, the active material layer is damaged and peels off from the current collector to deteriorate the cycle characteristics of the battery.

It is known to use a polyimide resin excellent in mechanical strength as a binder in the negative electrode comprising the silicon-based material to solve this problem. Patent document 1 discloses that the cycle characteristics of the batteries are improved with a binder having the mechanical characteristics of a tensile strength of 50 N/mm2or more, an elongation at break of 10% or more, a strain energy density of 2.5×10−3J/mm3or more and a coefficient of elasticity of 10000 N/mm2or less. In addition, Patent document 1 discloses that polyimide binders have these mechanical characteristics and are preferred as a binder of the electrodes comprising the silicon-based material.

CITATION LIST

Patent Document

Patent Document 1: International Publication WO No. 2004/004031

SUMMARY OF INVENTION

Technical Problem

However, the polyimide binders having high mechanical strength, described in the above prior art document, are obtained by heat-treating a polyamic acid in an electrode mixture paste, and in this heat treatment, the electrode mixture paste has to be exposed to high temperature such as 400° C. This is because the reaction from polyamic acid to polyimide does not proceed in the heat treatment at low temperature, causing problems such as that cycle characteristics of the produced battery decrease. The heat treatment at high temperature increases the manufacturing cost of the battery and requires heating equipment to expose the electrodes to high temperature conditions, and thereby, the heat treatment at lower temperature has been desired.

An object of the present invention is to provide a binder composition for a secondary battery for lowering temperature in the heat treatment of the polyimide binder, which is the above mentioned problem.

Solution to Problem

The binder composition for a secondary battery of the present invention comprises a polyamic acid and an aromatic compound comprising an electron donating group and an organic acid group.

Advantageous Effect of Invention

According to the binder composition for a secondary battery of the present invention, it is possible to provide a polyimide binder capable of providing excellent cycle characteristics to the produced battery even if the heat treatment is conducted at lower temperature. In addition, the present binder can be used as a resin composition not only for a secondary battery but also for an adhesive and a molded product. That is, according to the present resin composition or the resin composition further comprising filler, a polyimide resin which can be prepared by the heat treatment at lower temperature can be provided.

