In recent years, rechargeable and dischargeable electric storage devices including nonaqueous electrolyte cells represented by lithium ion cells and capacitors such as electric double layer capacitors and the like have been used as power sources for electronic equipment whose performance enhancement and downsizing have been progressed, power sources for power storage, power sources for electric vehicles and the like.
The nonaqueous electrolyte cell is a cell in which a negative electrode and a positive electrode prepared by providing a negative electrode active material layer (negative electrode layer) and a positive electrode active material layer (positive electrode layer) on a current collector composed of a metal foil, respectively, are arranged to face each other through a separator electrically separating the electrodes, and ions are accepted and donated between the positive electrode and negative electrode in a nonaqueous electrolyte to charge and discharge the cell.
Carbon materials are used as the negative electrode active material because of their enhanced safety and enhanced cycle characteristics; and as such carbon materials, carbon materials having a large specific surface area are used in order to obtain a high energy density.
For example, Patent Document 1 to Patent Document 3 (Patent Document 1: JP-A-2009-158532, Patent Document 2; JP-A-2007-39289, and Patent Document 3; JP-A-2010-21032) each describe an electrode assembly using a negative electrode active material composed of a carbon material having a plurality of pores in its surface.
Patent Document 4 (Patent Document 4: JP-A-2000-138061) describes a method of increasing the specific surface area of a negative electrode layer by applying and drying and thereafter pressurizing a slurry containing a graphite material having its surface coated with an amorphous carbon to a current collector.
However, it is difficult to effectively improve the power output of a cell only by increasing the specific surface area of a negative electrode layer. For example, the method of increasing the specific surface area by pressurizing a negative electrode layer containing graphite as described above results in formation of a large number of pores having a diameter of 1 nm or smaller when the pressure during application is too strong. If excessively fine pores like this occur in large amount in the graphite, there arises a possibility of causing a decrease in the output.