Suppressed shuttle effect and Self-Discharge of redox electrochemical capacitors using biphasic electrolyte with a Liquid-Liquid interface

Redox-enhanced electrochemical capacitors (redox ECs) can boost the energy density of conventional ECs by introducing soluble redox species in the electrolyte to gain additional capacity via faradaic reactions. The shuttle effect of the added redox species such as polyiodides, however, leads to rapid self-discharge of redox ECs and severely limits their capability for long-term energy storage. Herein, we propose an ionic liquid (IL)-based aqueous biphasic system (ABS) as the electrolyte of iodide-based redox ECs to suppress the self-discharge. Based on molecular dynamics (MD) simulation and mechanistic analysis, we reveal that the liquid-liquid interface in the ABS blocks the diffusion of polyiodides and leads to reduced self-discharge rate. Specifically, EC cells with ABS electrolyte deliver much lower open circuit voltage (OCV) drop (0.7 V) and better energy retention (>32%) in 24 h than that of cells with conventional iodide-based electrolyte (OCV drop: 1.6 V, energy retention: 0%). This strategy is also successfully extended to the fabrication of ABS-based hydrogel electrolyte for solid-state redox ECs with low self-discharge rate. The results of this work not only provide a mechanistic approach to elucidating the effect of redox couples on the self-discharge of redox ECs, but also demonstrate an effective way of mitigating the shuttle effect via liquid-liquid interface in a biphasic electrolyte to reduce self-discharge.