次世代大容量キャパシタの展望

抄録

Electrochemical capacitors use activated carbons for both positive and negative electrodes that show a non-faradaic, double-layer charge-discharge mechanism in a symmetric configuration. Thus electrochemical capacitors are efficient energy storage devices that exhibit long lifespans and extremely rapid charge-discharge characteristics compared with batteries. Today, capacitor technology is regarded as promising and has an additional advantage with increasing effectiveness when combined with solar and wind regenerative energy sources. In recent years, composite battery materials have been vigorously researched in the hope of improving their energy density. Hybridizing battery and capacitor materials overcomes the energy density limitation of existing generation-I capacitors without much sacrifice of the cycling performance. Normal battery-capacitor hybrids use a high-energy and sluggish redox electrode and low-energy and fast double-layer electrodes, possibly producing a larger working voltage and higher overall capacitance. In order to smoothly operate such asymmetric systems, however, the rates of the two different electrodes must be highly balanced. Especially, the redox rates of the battery electrodes must be substantially increased to the levels of double-layer process. In this perspective we summarize various hybrid systems and show representative aqueous and non-aqueous asymmetric configurations for their energy-power improvement. We attempt to identify the essential issues for the realizable hybrids and suggest ways to overcome the rate increase by exemplifying ultrafast performance of the Li₄Ti₅O₁₂ nanocrystal prepared by a unique in-situ material processing technology under ultra-centrifugation.