Surface engineered covalent bridging strategy to in-situ fabricate MXene-based core-sheath fiber for flexible supercapacitors with ultrahigh volumetric energy density and mechanical properties

MXene fiber-based supercapacitors (SCs) are expected to make a major contribution to the ongoing development of wearable electronics and metaverse technologies in future. However, to fabricate fiber electrodes with high energy density and strong mechanical properties remains a major challenge for their usage in SCs. Herein, we fabricated a flexible core-sheath structural Ti₃C₂T_x MXene@polyaniline (MX@PA) fiber electrode with ultrahigh volumetric energy density and good tensile strength through surface engineered covalent bridging strategy via wet spinning and in situ oxidant-free polymerization processes. Benefiting from the high-order core-sheath structure and strong Ti-O-N covalent bonds between MXene and PANI, an ultrahigh tensile strength (∼168.1 MPa) and super-toughed (∼1.75 MJ cm⁻³) fiber electrode was achieved. The assembled SC provided much higher volumetric capacitance of 631F cm⁻³ (based on the SC device) at a current density of 0.5 A cm⁻³ and ultralong-term cycling stability with 92.6 % capacitance retention after 10000 cycles due to the rapid electron conduction of core (MXene) and large pseudocapacitive charge transfer of the sheath (PANI). As a result, the SC delivers excellent volumetric energy density of 56.1 mWh cm⁻³ at a power density of 204.9 mW cm⁻³. The integrated SC found actual application to power a 2.5 V red LED.