Ultrahigh-energy-density supercapacitors based on all-pseudocapacitive binary metal sulfide-MXene composites

MXenes, a family of two-dimensional (2D) transition metal carbides and nitrides, have gained much attention for use as promising electrode materials for supercapacitors (SCs) owing to their metallic conductivities and reliable electrochemical performances. However, since they are prone to oxidation at anodic potentials, the fabrication of ultra-high energy density SCs utilizing both an MXene-based cathode and anode remains a great challenge. Here, we successfully incorporated pseudocapacitive FeZnS and MnZnS nanoparticles into Ti₃C₂T_x MXene for use as an MXene-based cathode (c-Mx) and anode (a-Mx), respectively. The fabricated c-Mx and a-Mx exhibit higher gravimetric capacitance and rate performance than pristine Ti₃C₂T_x because of the numerous pseudocapacitive reaction sites and increased d-spacing of Ti₃C₂T_x arising from the incorporation of metal sulfide nanoparticles. Notably, a-Mx exhibits stable electrochemical behavior even at anodic potentials. SCs fabricated with c-Mx and a-Mx yielded outstanding energy-storage performances, including high specific capacitance (366.4 F g⁻¹ at 1 A g⁻¹), ultrahigh energy density (130.27 W h kg⁻¹ at a power density of 800.0 W kg⁻¹), and excellent cycle stability (>6000 cycles). This is attributed to the high conductivity of MXenes, which enables effective pseudocapacitive reactions of FeZnS and MnZnS, as well as the well-matched charge balance between c-Mx and a-Mx.