Self-templated formation of (NiCo)₉S₈yolk-shelled spheres for high-performance hybrid supercapacitors

Rational materials design for the synthesis of desirable hollow micro- A nd nanostructures has recently revealed the remarkable potential for high-performance energy storage and conversion devices. Owing to their unique "core-void-shell"structural configurations, yolk-shell-structured electrode materials can achieve intimate contact with the electrolyte and alleviate the volume expansion issue during electrochemical cycling, which is therefore poised to further boost the electrochemical properties of hybrid supercapacitors. Herein, a facile self-templated strategy, consisting of a hydrothermal step and a high-temperature sulfurization process, has been developed for the construction of yolk-shell (NiCo)9S8 spheres in situ coated by graphite carbon ((NiCo)9S8/GC) due to the non-equilibrium thermal treatment of alkali metal alkoxides. The as-synthesized yolk-shelled sphere exhibits a high specific capacitance of 1434.4 F g-1 (179.3 mA h g-1) at a current density of 1 A g-1, and good rate capability and cycling stability with 83.1% capacitance retention at 8 A g-1 over 5000 cycles. To further demonstrate its practical application, a hybrid supercapacitor device was assembled using (NiCo)9S8/GC as the battery-type positive electrode and activated carbon (AC) as the capacitive-type electrode. The as-fabricated device can reach a wide voltage window of up to 1.6 V, deliver a high energy density of 55.6 W h kg-1 at a power density of 800.3 W kg-1 and maintain 90.2% of specific capacitance after 3000 cycles.