2D-on-2D core-shell Co₃(PO₄)₂stacked micropetals@Co₂Mo₃O₈nanosheets and binder-free 2D CNT-Ti₃C₂T_{: X} -MXene electrodes for high-energy solid-state flexible supercapacitors

The structural instability and sluggish kinetics of conventional positive electrodes with the lower capacitance of carbon-based negative electrodes result in an inferior performance for state-of-art supercapacitors (SCs). A general yet sustainable approach is proposed here to overcome this hitch by assembling hybrid SC cells utilising porous and stable 2D-on-2D core-shell and carbon/pseudocapacitive composite electrodes. Porous Co3(PO4)2 transparent stacked micropetals (TSMs) were synthesised and decorated with Co2Mo3O8 nanosheets (NSs) (Co3(PO4)2@Co2Mo3O8) forming a 2D-on-2D core-shell positive electrode, which was combined with a 2D carbon nanotube/MXene (CNT-Ti3C2TX) composite negative electrode. The core-shell electrode achieved a specific capacity of 184.7 mA h g-1 (738 mF cm-2) and cycling stability of 95.6% over 15 000 charge/discharge cycles. The CNT-Ti3C2TX electrode exhibited a remarkable areal capacitance of 187.5 mF cm-2 and cycling stability of 93.1%. Consequently, the assembled unique hybrid solid-state SCs delivered an exceptional volumetric capacitance of 7.9 F cm-3 and a specific energy of 74.06 W h kg-1 (2.47 mW h cm-3) at a specific power and cycling stability of 1.13 kW kg-1 and 93.2%, respectively. Overall, the techniques and electrode materials presented in this study can serve as a reference to produce a range of electrode materials for next-generation energy storage devices.