High-capacity Bi2O3 anode for 2.4 V neutral aqueous sodium-ion battery-supercapacitor hybrid device through phase conversion mechanism

Aqueous battery-supercapacitor hybrid devices (BSHs) are of great importance to enrich electrochemical energy storage systems with both high energy and power densities. However, further improvement of BSHs in aqueous electrolytes is greatly hampered by operating voltage and capacity limits. Different from the conventional intercalation/de-intercalation mechanism, Bi2O3 implements charge storage by a reversible phase conversion mechanism. Herein, taking Bi2O3 electrode with wide potential window (from −1.2 to 1 V vs. saturated calomel electrode) and high capacity as battery-type anode, we propose that the overall performance of aqueous BSHs can be greatly upgraded under neutral condition. By paring with stable layer-structured δ-MnO2 cathode, a sodium-ion Bi2O3//MnO2 BSH with an ultrahigh voltage of 2.4 V in neutral sodium sulfate electrolyte is developed for the first time. This hybrid device exhibits high capacity (~215 C g−1 at 1 mA cm−2), relatively long lifespan (~77.2% capacity retention after 1500 cycles), remarkable energy density (71.7 Wh kg−1@400.5 W kg−1) and power density (3204.3 W kg−1@18.8 Wh kg−1). Electrochemical measurements combining a set of spectroscopic techniques reveal the reversible phase conversion between bismuth oxide and metallic bismuth (Bi2O3 ⇋ Bi0) through Bi2+ transition phase in neutral sodium sulfate solution, which can deliver multielectron transfer up to 6, leading to the high-energy BSHs. Our work sheds light on the feasibility of using Bi2O3 electrode under neutral condition to address the issue of narrow voltage and low capacity for aqueous BSHs.