Self-discharge in flowless Zn-Br2 batteries and its mitigation

Abstract

Several decades after the invention of the flow Zn-Br2 systems, persistent attempts have been made to develop stationary Zn-Br2 batteries. Such development should increase the energy density of the system simultaneously significantly reducing their cost and opening new challenges associated with the cell design and its performance. One of the major concerns is the rapid self-discharge of stationary systems leading to spontaneous charge loss during battery storage time. While self-discharge in flow cells is generally attributed to the chemical oxidation of the Zn anode, we show that the origin of self-discharge in a static configuration is completely different. By systematic investigations of activated carbon with different surface areas under varied charging conditions, mechanistic insights into this phenomenon were provided. Based on this understanding, we proposed herein an effective way to suppress the cathode's self-discharge by encapsulation of a bromine complexing agent inside the electrode's pore matrix. The modified electrodes unveil unique chemical kinetics of deaggregation of the stable fuse salt phase composed of bromine complexing agent and Br3− anions upon discharging. To the best of our knowledge, such a phenomenon has not been reported yet. The proposed system demonstrates high capacity (up to 300 mAh/g) and impressive long-term stability.