In recent years, lithium ion secondary batteries have been used as batteries for small devices such as notebook PCs, mobile phones, electric tools, or electronic/communication devices. In addition, recently, lithium ion secondary batteries have begun to be used as batteries for large devices such as electric vehicles or hybrid vehicles.
A lithium ion secondary battery is mainly composed of: a positive electrode that has a metal oxide such as a lithium cobalt composite oxide as an active material; a negative electrode that has a carbon material such as graphite as an active material; and an electrolytic solution in which a lithium salt is dissolved. This battery is charged or discharged by lithium ions moving between the positive electrode and the negative electrode.
The positive electrode is obtained by coating a slurry containing the metal oxide and a binder on a surface of a positive electrode current collector such as an aluminum foil and drying the slurry. The negative electrode is obtained by coating a slurry containing the carbon material and a binder on a surface of a negative electrode current collector such as a copper foil and drying the slurry. Each of the binders used at this time has a function of binding active material particles to each other and binding the active material and the current collector to each other to prevent the active material from being separated from the current collector.
Currently, polyvinylidene fluoride (PVDF) has been used as a binder. However, in the case of PVDF, binding strengths between active material particles and between an active material and a current collector are weak. Therefore, in order to obtain a sufficient binding strength, it is necessary that an electrode contain a large amount of PVDF. As the content of PVDF is increased, the amount of an active material contained in a battery is decreased. Therefore, the capacity and the energy density of the obtained lithium ion secondary battery are decreased. In addition, during the preparation of an electrode, since PVDF is dissolved in N-methylol pyrrolidone (NMP) to be used, expensive NMP is used in a large amount. Therefore, since PVDF is used as a binder, the manufacturing cost of a lithium ion secondary battery is increased. Further since an organic solvent is used, the work environment during the preparation of an electrode deteriorates.
As a method to solve these problems, a styrene-butadiene rubber (SBR)-based aqueous dispersion is disclosed in which carboxymethyl cellulose (CMC) is used as a thickener in combination with a binder (for example, refer to Patent Document 1). This SBR-based dispersion is inexpensive because an expensive organic solvent is not used. In addition, since the SBR-based dispersion is aqueous, there is an advantageous effect from the viewpoint of the work environment. Further, since SBR is superior in binding properties between active material particles and between an active material and a current collector, the content of a binder in an electrode can be decreased.
For the above-described reasons, SBR has been widely used as a binder for a lithium ion secondary battery electrode.
However, this SBR is poor in electrolytic solution resistance. In addition, when the SBR is in contact with an electrolytic solution in a battery for a long time, binding strengths between active material particles and between an active material and a current collector are decreased, and distances between the active material particles, between conductive auxiliary agent particles, and between the active material and the current collector are increased, respectively. As a result, the electron conductivity of an electrode is decreased, and the internal resistance of the battery is increased. As the temperature of the battery is increased, such a decrease in binding strength is likely to occur. Therefore, when a lithium ion secondary battery using SBR is used particularly at a high temperature, the internal resistance of the battery is increased, and charge-discharge characteristics are decreased.
Patent Document 2 discloses (A) a binder containing polymer particles obtained by polymerizing: (a) 20 to 79 parts by weight of a (meth)acrylic acid ester monomer; (b) 20 to 60 parts by weight of a vinyl monomer; and (c) 0.01 to 30 parts by weight of an unsaturated carboxylic acid monomer, based on 100 parts by weight of a binder polymer.
In addition, Patent Document 3 discloses a binder composition for a nonaqueous secondary battery, the binder composition including: functional group-containing resin fine particles (A); and a cross-linking agent (B), in which the functional group-containing resin fine particles (A) are obtained by emulsion polymerization of ethylenically unsaturated monomers, which contains a keto group-containing ethylenically unsaturated monomer, in water with a radical polymerization initiator in the presence of a surfactant, and the cross-linking agent (B) is a polyfunctional hydrazide compound.
Since the binders disclosed in Patent Documents 2 and 3 are superior in electrolytic solution resistance, charge-discharge characteristics at a high temperature are superior.