All Solid-State Coaxial Supercapacitor with Ultrahigh Scan Rate Operability of 250 000 mV/s by Thermal Engineering of the Electrode–Electrolyte Interface

Solid-state supercapacitors have never been able to compete with their liquid electrolyte counterparts, forming a major impediment for their utilization in portable and wearable electronics. Attempts to improve the rate capability of solid-state supercapacitors have predominantly focused on the morphology or porosity of the electrode material, and largely ignored the critical role of electrolyte. Here, we report the fabrication of a carbon nanotube yarn (CNT-yarn) based flexible all-solid-state coaxial-type supercapacitor operable at scan rates as high as 250 000 mV/s, exhibiting high energy (6.2 mWh/cm3) and power density (4465 mW/cm3). Electrode–electrolyte interfacial resistance is lowered by 28.3% to achieve ultrafast frequency response (τ = 3.2 ms) through thermal engineering of the CNT-yarn–polymer electrolyte interface. This creates synergistic chemical functionality on the CNT-yarn and simultaneous diffusional broadening of the electrode–electrolyte interface, as revealed by micro-Raman spectral mapping, and accounts for both the high rate capability and high energy density. High Columbic efficiency (∼98%) and extremely low iRdrop (<5%) that is unprecedented among solid-state supercapacitors are direct implications of such thermal interfacial engineering. Furthermore, the coaxial device is mechanically tenacious and bendable up to 360°, with superior cyclability (95% for 10000 cycles) as demonstrated by its seamless integration on to wearable platforms.