Reinforced supercapacitive behavior of O₃-type layer-structured Na₃Ni₂BiO₆ in 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF₄) electrolyte

Sodium-ion-based energy storage devices in which the electrochemical properties are influenced by excess Na ions are very important as an alternative energy storage system. In this study, O₃-type multi-metal-oxide layers of Na₃Ni₂BiO₆ were synthesized via a simple solid-state method. Their electrochemical properties were examined while employing them as a supercapacitor electrode material with four different electrolytes: H₂SO₄, Na₂SO₄, KOH, and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF₄). Among these, BMIMBF₄ ionic liquid electrolyte in a symmetric Na₃Ni₂BiO₆ supercapacitor demonstrated the highest specific capacitance performance of 780.49 F g⁻¹ at 0.5 A g⁻¹ with 95.2% capacitance retention after 10,000 cycles. Moreover, a maximum energy density of 67.16 Wh·kg⁻¹ was recorded at 349.87 W·kg⁻¹. The enhanced electrochemical performance is a result of the reinforced fast intercalation/deintercalation of Na⁺ ions in the Na₃Ni₂BiO₆ crystal structure due to high solvation and association mechanism of Na₃Ni₂BiO₆ in BMIMBF₄ ionic liquid media. These results indicate that the developed Na₃Ni₂BiO₆-based supercapacitor with BMIMBF₄ ionic liquid electrolyte could be used in the high-performance energy storage devices for smart electrical and electronic units.