Na 2 [(VO) 2 (HPO 4 ) 2 (C 2 O 4 )]·2H 2 O: A Promising Mixed Polyanionic Cathode Material for Aqueous Zn-Ion Batteries.

A cathode material, Na 2 [(VO) 2 (HPO 4 ) 2 (C 2 O 4 )]·2H 2 O, for aqueous zinc-ion batteries is synthesized by a hydrothermal method. Na 2 [(VO) 2 (HPO 4 ) 2 (C 2 O 4 )]·2H 2 O presents a two-dimensional layered structure and sheetlike morphology, which provide fast and reversible Zn 2+ insertion/extraction. Na 2 [(VO) 2 (HPO 4 ) 2 (C 2 O 4 )]·2H 2 O delivers a high average voltage plateau (∼1.3 V), a moderate specific capacity (90 mA h g -1 ), and good cycle stability (100% after 100 cycles). The Zn 2+ storage mechanism is investigated by in situ XRD and ex situ XPS.