Pseudocapacitance of TiO_2−x/CNT Anodes for High‐Performance Quasi‐Solid‐State Li‐Ion and Na‐Ion Capacitors

Abstract It is challenging for flexible solid‐state hybrid capacitors to achieve high‐energy‐high‐power densities in both Li‐ion and Na‐ion systems, and the kinetics discrepancy between the sluggish faradaic anode and the rapid capacitive cathode is the most critical issue needs to be addressed. To improve Li‐ion/Na‐ion diffusion kinetics, flexible oxygen‐deficient TiO_2−x/CNT composite film with ultrafast electron/ion transport network is constructed as self‐supported and light‐weight anode for a quasi‐solid‐state hybrid capacitor. It is found that the designed porous yolk–shell structure endows large surface area and provides short diffusion length, the oxygen‐deficient composite film can improve electrical conductivity, and enhance ion diffusion kinetic by introducing intercalation pseudocapacitance, therefore resulting in advance electrochemical properties. It exhibits high capacity, excellent rate performance, and long cycle life when utilized as self‐supported anodes for Li‐ion and Na‐ion batteries. When assembled with activated carbon/carbon nanotube (AC/CNT) flexible cathode, using ion conducting gel polymer as the electrolyte, high energy densities of 104 and 109 Wh kg⁻¹ are achieved at 250 W kg⁻¹ in quasi‐solid‐state Li‐ion and Na‐ion capacitors (LICs and SICs), respectively. Still, energy densities of 32 and 36 Wh kg⁻¹ can be maintained at high power densities of 5000 W kg⁻¹ in LICs and SICs.