Lithium ion secondary batteries, nickel-cadmium storage batteries, and nickel hydride storage batteries are widely used as power sources for portable information-communication terminals including cellular phones and notebook-size personal computers, and for video cameras and portable music players, or the like. Among the batteries, the lithium ion secondary batteries, which are superior in properties such as high energy density and high output density, have been rapidly researched and developed since the debut thereof, and have established themselves as standard batteries for these consumer appliances.
With increase in functionality of the portable information-communication terminals, the lithium ion secondary batteries (hereinafter also simply referred to as “batteries” (or “a battery”)) serving as power sources require further higher energy densities, i.e., require further higher capacities. In addition, the batteries require longer cycle lives in consideration of environmental standpoints.
In general, a lithium ion secondary battery includes a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte solution. For example, the positive electrode and the negative electrode are obtained by mixing an active material, a conductive material for imparting electrical conductivity, and a binder for binding these components in a solvent, and applying the mixture to a current collector including a metal foil or the like.
For example, as described in PTL 1, polyvinylidene fluoride (PVDF) which is less likely to be deteriorated even when it is brought into contact with an electrolyte solution is conventionally used as a binder. However, when PVDF is used as the binder, adhesion with the current collector is insufficient, which makes it difficult to improve a cycle life. Consequently, in these days, styrene-butadiene rubber and a polyimide-based resin which can achieve high adhesion with the current collector are used as the binder.