Fast Sodium Ion Tracking at the Battery Electrode Interface During Cycling in Concentrated Aqueous Electrolytes

Ion transport at the electrode–electrolyte interface is key for attaining high rate performance in next‐generation batteries. In operando tracking of this process is challenging, especially in the high electrolyte concentrations utilized for emerging aqueous batteries. Only recently, our group demonstrated the usage of microelectrode voltammetry with ferrocyanide (FeCN) to access high electrolyte concentrations in aqueous potassium ion batteries, but the poor temporal resolution limited knowledge of the ion dynamics occurring at short times during charge/discharge. Herein, we target aqueous sodium ion battery electrolytes (up to 3 mol dm −3 ) using high voltammetry scan rates to attain improved temporal resolution. Changes in the measured E 1/2 of FeCN oxidation showed high linearity with log changes in [Na + ] and could be combined with scanning electrochemical microscopy (SECM) for tracking of concentration changes during sodium insertion and extraction. Through our methods, we could successfully observe the evanescent formation of ion concentration gradients during cycling of an organic composite electrode. Furthermore, the time evolution of the ion transfer processes suggests differences in kinetics and composition at various locations across the battery electrode surface. Overall, this work progresses ion sensors for concentrated electrolytes and the characterization of rapid ion transfer events in batteries.