Selective Zn/Na Ions Insertion into FePO4 Positive Electrode Tuned by Counter Anions in Aqueous Zn‐Based Rechargeable Batteries

Abstract Toward the construction of rechargeable batteries utilizing Zn metal has a high theoretical capacity, one of the solutions is to combine with the insertion/deinsertion of Na ions into/from hosts, but in the presence of two coexisting metal cations in dual‐salt electrolytes, it has been often left to spontaneity to determine which cations the host materials react with preferentially. The hybrid aqueous electrolytes containing 1 m Zn‐salt and 6 m Na‐salt were used, and the counter anions were TFSA − and Cl − , respectively. In the TFSA‐based electrolyte, the ligands for Zn ions and Na ions are water molecules to form positively‐charged hydrated cations. In contrast, for the Cl‐based electrolyte, Zn ions undergo partially solvation of Cl − to be stabilized as ZnCl 4 (H 2 O) 2 2− with negative charges while Na ions are predominantly hydrated. The galvanostatic discharge/charge tests, AC impedance measurements, and field‐emission electron probe micro analyzer revealed that in the Zn(TFSA) 2 /NaTFSA system, where both the cations are positively charged, Zn ions with higher charge density were inserted into the FePO 4 . On the other hand, in the ZnCl 2 /NaCl system, the FePO 4 accommodated Na ions positively charged in the electrolyte solution rather than negatively charged Zn ions.