Operational strategy for multiredox-reaction electrodes for long-lifespan Na-ion hybrid capacitors

The development of high-performance sodium storage materials is becoming necessary owing to the limited availability of lithium. Materials undergoing a multiredox-reaction mechanism are highly promising for sodium-ion batteries (SIBs) and sodium-ion hybrid capacitors (SICs). In this study, we propose an operational strategy for improving the cycling lifespan of multiredox-reaction materials in SIBs and SICs. The efficiency of this strategy was evaluated using a Na2VTi(PO4)3@C (NVTP@C) electrode with three redox reaction sites (V2+/V3+, Ti3+/Ti4+, and V3+/V4+). Electrochemical studies revealed that the exclusion of the most unfavorable reaction of NVTP@C (i.e., the V2+/V3+ redox reaction) is effective in reducing the voltage hysteresis of Na||NVTP@C half-cells and NVTP@C||AC hybrid ion capacitor systems, thereby extending their cycling life. Thus, based on a fundamental understanding of multiredox materials, our strategy can be used to design advanced SIBs and SICs based on multiredox-reaction mechanisms.