As electronic devices become smaller and lighter in weight with greater portability, research and development continue toward secondary batteries with higher voltage and higher energy density characteristics. The rapid advancement in performance of recent portable electronic devices is leading, in particular, to comparable rapid increase in their power consumption. A demand therefore exists for secondary batteries that can realize even higher voltage and higher energy density.
Such secondary batteries have led to demand for ionic conductive electrolytes with high withstand voltage, that exhibit high ionic conductivity and are electrochemically stable in a wide potential range, and therefore numerous attempts have been made recently to utilize ambient temperature molten salts that are flame retardant and noncombustible, in electrochemical devices such as secondary batteries and electrical double layer capacitors. (For example, see Japanese Unexamined Patent Publication No. 2002-110230).
One common method for fabricating secondary battery electrodes is a method in which a solution of a binder material in a polar organic solvent is prepared first, active materials and other electrode components are added to the solution to prepare a slurry, and the slurry is coated and dried onto a metal foil as the current collector to form an active material layer, which is then roll pressed. When the solvent is removed by drying in this method, the active material layer can become non-uniform due to, for example, voids produced at the sections where the solvent was present. When a secondary battery is fabricated using such electrodes, a separator is sandwiched between the positive electrode and negative electrode to prevent shorting, and when the electrolyte solution is injected therein to form a secondary battery it is important for the electrolyte solution to penetrate through the electrodes. However, electrolytes obtained using ambient temperature molten salts, for example, have high viscosity and penetrate poorly into electrodes, thus making it difficult to obtain satisfactory charge-discharge characteristics for the secondary battery.