An Aqueous Symmetrical Sodium-Ion Battery Using New Concentrated Sodium Trifluoroacetate Electrolyte

摘要

Aqueous Na-ion batteries have gained increasing attention as alternative devices to conventional nonaqueous Li-ion batteries for larger-scale energy storage, because they face formidable problems on manufacturing cost, rate capability, and safety. Although thermodynamic theoretical electrochemical window of water (1.23 V) has imposed a restriction on aqueous batteries in the selection of cathode and anode active materials, recent studies have revealed that some highly concentrated electrolytes effectively prevent electrochemical decomposition of water, and that a judicious choice of electrode active materials with flat potential plateaus within the range of a kinetic practical electrochemical window successfully leads to the expansion of working voltage in aqueous cell. Nevertheless, the solute for reported highly concentrated electrolytes has been limited to either highly expensive perfluoroalkylsulfonylates or perchlorates that require special care in handling under dry condition. We have explored more beneficial solutes that contribute in the reduction of manufacturing cost and found that a concentrated aqueous solution of NaCO_2CF_3 (NaTFA, sodium trifluoroacetate) serves as a suitable electrolyte when NASICON-type Na_2VTi(PO_4)_3 (NVTP) is employed both as cathode (V~(3+)/V~(4+)) and anode (Ti~(4+)/Ti~(3+) and V~(3+)/V~(2+)). Herein we discuss on the symmetrical operation of NVTP with very flat plateaus in this aqueous Na-ion battery, whose capacity retention is highly dependent on the choice of electrolytes.