Abstract: To overcome the limited electronic conductivity and capacity of single and binary transition metal phosphates (TMPs), highly electrochemical active materials and rational structural design of ternary TMPs composite are urgently required. In this study, we successfully synthesized an amorphous 3D Ni-Co-Mn phosphate@2D Ti₃C₂T_x (MXene) nanocomposite (NCMP series) through the electrodeposition method. The amorphous Ni-Co-Mn phosphate effectively restricts the self-accumulation of MXene nanosheets, resulting in the development of a porous nanostructure. This structure exposes more active sites, expands the ion transport path, and enhances the conductivity of the Ni-Co-Mn phosphate@Ti₃C₂T_x material. Owing to the synergistic effect offered by Ni-Co-Mn phosphate and MXene nanocomposite, the anchored Ni-Co-Mn phosphate@Ti₃C₂T_x (NCMP-5) electrode delivers an elevated capacity of 342 mAh/g (1230 C/g) at 5.0 A/g, surpassing the pristine Ni-Co-Mn phosphate (NCMP-4, 260 mAh/g) and MXene (33.3 mAh/g). Moreover, a hybrid solid-state supercapacitor (HSSC) device is assembled with NCMP-5 as a cathode and reduced graphene oxide (rGO) as an anode within a polymer gel (PVA-KOH) electrolyte. Notably, the fabricated HSSC device displays a supreme specific capacity of 27.5 mAh/g (99 C/g) and a high (volumetric) energy density of 22 Wh/kg (3.6 Wh/cm³) at a power density of 0.80 kW/kg (0.13 kW/cm³) for 1.0 A/g. Moreover, the HSSC device retains 95.4 % of its initial capacity even after 10,000 cycles. Importantly, the operational potential window of two serially connected HSSC devices approaches +3.2 V, enabling different colored commercial light-emitting diodes (LEDs) to be efficiently illuminated. Eventually, the remarkable supercapacitive characteristics of the 3D@2D amorphous Ni-Co-Mn phosphate@MXene nanocomposite make it an attractive choice for advanced electroactive materials in upcoming hybrid energy storage technologies.

Key words: Ni-Co-Mn phosphate, Ti₃C₂T_x, Electrodeposition, Nanocomposite, Amorphous nature, Hybrid solid-state supercapacitor