Lithium-ion capacitor

A lithium-ion capacitor (LIC) is a hybrid type of capacitor classified as a type of supercapacitor. Activated carbon is typically used as the cathode. The anode of the LIC consists of carbon material which is pre-doped with lithium ions. This pre-doping process lowers the potential of the anode and allows a relatively high output voltage compared with other supercapacitors. In 1981, Dr. Yamabe of Kyoto University, in collaboration with Dr. Yata of Kanebo Co., created a material known as PAS (polyacenic semiconductive) by pyrolyzing phenolic resin at 400–700 °C. This amorphous carbonaceous material performs well as the electrode in high-energy-density rechargeable devices. Patents were filed in the early 1980s by Kanebo Co., and efforts to commercialize PAS capacitors and lithium-ion capacitors (LICs) began. The PAS capacitor was first used in 1986, and the LIC capacitor in 1991. A lithium-ion capacitor is a hybrid electrochemical energy storage device which combines the intercalation mechanism of a lithium-ion battery anode with the double-layer mechanism of the cathode of an electric double-layer capacitor (EDLC). The packaged energy density of an LIC is approximately 20 Wh/kg, roughly four times higher than an EDLC and five times lower than a lithium-ion battery. The power density, however, has been shown to match that of EDLCs, as it is able to completely discharge in seconds. At the negative electrode (cathode), for which activated carbon is often used, charges are stored in an electric double layer that develops at the interface between the electrode and the electrolyte. The positive electrode (anode) was originally made from lithium titanate oxide, but is now more commonly made from graphitic carbon to maximize energy density. The graphitic electrode potential initially at -0.1 V versus SHE (standard hydrogen electrode) is lowered further to -2.8 V by intercalating lithium ions. This step is referred to as 'doping' and often takes place in the device between the anode and a sacrificial lithium electrode. The pre-doping process is critical to the device functioning as it can significantly affect the development of the solid electrolyte interphase (SEI) layer. Doping the anode lowers the anode potential and leads to a higher output voltage of the capacitor. Typically, output voltages for LICs are in the range of 3.8–4.0 V but are limited to minimum allowed voltages of 1.8–2.2 V. If the voltage drops any lower than this lithium ions will deintercalate more rapidly than they can be restored during normal use. Like EDLCs, LIC voltages vary linearly adding to complications integrating them into systems which have power electronics that expect the more stable voltage of batteries. As a consequence, LICs have a high energy density, which varies with the square of the voltage. The capacitance of the anode is several orders of magnitude larger than that of the cathode. As a result, the change of the anode potential during charge and discharge is much smaller than the change in the cathode potential. Other candidate anode materials are being investigated as alternative to graphitic carbons, such as hard carbon, soft carbon, graphene-based carbons. The expected benefit compared to graphitic carbons is in increasing the doped electrode potential improving power capability as well as safety in regard to metal plating.