Operational Strategy for Multiredox-Reaction Electrodes for Long-Lifespan Na-Ion Hybrid Capacitors

Abstract

High-performance sodium storage materials are becoming necessary owing to the limited availability of lithium. In particular, materials that exhibit a multiredox reaction mechanism are considered highly promising for sodium-ion batteries (SIBs) and sodium-ion hybrid capacitors (SICs). In this paper, we propose an operational strategy for improving the cycling lifespan of multiredox-reaction materials in SIBs and SICs. 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 of NVTP@C 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 both Na||NVTP@C half-cells and NVTP@C||AC hybrid ion capacitor systems and hence extending their cycling life. Thus, our strategy, which is based on a fundamental understanding of multiredox materials, can be used as a general design principle for advanced SIBs and SICs based on multiredox mechanisms.

Keywords: Multiredox-reaction, NASICON, Sodium-ion batteries, Sodium-ion capacitor, Hybrid-ion capacitor