Zinc Ion Hybrid Capacitors: Four Essential Parameters Determining Device Energy Density

Zinc ion hybrid capacitors (ZIHCs) with Zn metal faradic and carbon capacitive electrodes have potential applications in grid-scale energy storage systems and wearable devices. However, the high specific energy density reported in many recent studies is based on the mass of active carbon materials alone, with deficient device energy density. This perspective article discusses how four crucial parameters influence the device energy density of ZIHCs, including areal mass loading (m_c) and specific capacity (Q_g,c) of active carbon materials in cathodes, negative-to-positive electrode capacity ratio (N/P), and electrolyte-to-active carbon materials mass ratio (E/C). Using a representative device model, how the device energy density varies when these four parameters change is shown. Detailed analysis indicates that specific parameter windows with the four parameters within narrow ranges (e.g., m_c = 10-20 mg cm^-2, Q_g,c > 100 mAh g^-1, N/P < 20, and E/C < 5) need to be achieved simultaneously to deliver application-relevant energy density (e.g., >30 Wh kg^-1) in ZIHCs. It is hoped that these findings assist in objectively evaluating reported performance data and identifying essential issues for future research development to realize practical applications.