Controllable Fabrication and Tuned Electrochemical Performance of Potassium Co-Ni Phosphate Microplates as Electrodes in Supercapacitors

Most reported pristine phosphates, such as NH₄MPO₄·H₂O (M = Co, Ni), are not very stable as supercapacitor electrodes because of their chemical properties. In this work, KCo_xNi_1-xPO₄·H₂O microplates were fabricated by a facile hydrothermal method at low temperature and used as electrodes in supercapacitors. The Co and Ni content could be adjusted, and optimal electrochemical performance was found in KCo_0.33Ni_0.67PO₄·H₂O, which also possessed superior specific capacitance, rate performance, and long-term chemical stability compared with NH₄Co_0.33Ni_0.67PO₄·H₂O because of its unique chemical composition and microstructure. Asymmetric supercapacitor cells based on KCo_0.33Ni_0.67PO₄·H₂O and active carbon were assembled, which produce specific capacitance of 34.7 mA h g^-1 (227 F g^-1) under current density of 1.5 A g^-1 and retain 82% as initial specific capacitance after charging and discharging approximately 5000 times. The assembled asymmetric supercapacitor cells (ASCs) exhibited much higher power and energy density than most previously reported transition metal phosphate ASCs. The KCo_xNi_1-xPO₄·H₂O electrodes fabricated in this work are efficient, inexpensive, and composed of naturally abundant materials, rendering them promising for energy storage device applications.