1. Field of the Invention
The present invention relates to a technology that is effective when applied to an electric storage device.
2. Description of the Related Arts
A lithium ion secondary battery or a lithium ion capacitor has been proposed as an electric storage device that is mounted on an electric vehicle, hybrid vehicle, or the like (see JP-A-2006-286919). A lithium cobalt oxide or the like is contained in a positive electrode of the lithium ion secondary battery as a positive-electrode active material. A graphite or the like is contained in a negative electrode of the lithium ion secondary battery as a negative-electrode active material. An activated carbon is contained in a positive electrode of the lithium ion capacitor as a positive-electrode active material. A PAS or the like is contained in a negative electrode of the lithium ion capacitor as a negative-electrode active material. When these electric storage devices are charged or discharged, lithium ions move between the opposite electrodes.
A separator is provided between the electrodes of the electric storage device. The separator is impregnated with an electrolyte solution serving as a moving path of the lithium ions. Therefore, the lithium ions move between the surfaces of the electrodes that are in contact with the separator when the device is charged or discharged. However, there may be the case in which the lithium ions go into the end surfaces of the electrodes depending upon the magnitude of the charging current or discharging current.
As disclosed in JP-A-2006-286919, a metallic current collector is exposed at the end surface of the electrode of the electric storage device. Therefore, when the lithium ions go into the end surface of the electrode, the lithium ions are adsorbed onto the current collector on the end surface of the electrode. In this case, the current collector cannot take in the lithium ions so that the metal lithium might be deposited from the exposed surface of the current collector. The deposition of the metal lithium causes short-circuit between the electrodes, thereby deteriorating the safety of the electric storage device. Particularly, in a lithium ion capacitor or the like that aims to increase an output, it is important to prevent the lithium ions from going into the end surface of the electrode.