K3v2(Po4)3/C as a New High‐Voltage Cathode Material for Calcium‐Ion Batteries with a Water‐In‐Salt Electrolyte

Abstract Aqueous Ca‐ion batteries (ACIBs) attract immense attention due to its high safety and the high abundance of calcium. However, the development of ACIBs is hindered by the lack of high voltage cathode materials to host the large radius and divalent Ca 2+ . Herein, polyanionic phosphate K 3 V 2 (PO 4 ) 3 /C (KVP/C) is provided as a new cathode material for ACIBs. Due to the robust structure of polyanion material and the wide electrochemical window of water‐in‐salt electrolyte, KVP/C delivers a high working voltage of 3.74 V versus Ca 2+ /Ca with a specific capacity of 102.4 mAh g −1 and a long‐life of 6000 cycles at 500 mA g −1 . Furthermore, the calcium storage mechanism of KVP/C is shown to be the coexistence of solid solution and two‐phase reaction by in situ X‐ray diffraction, ex situ transmission electron microscope, and X‐ray photoelectron spectroscopy. Finally, an aqueous calcium‐ion full cell, based on an organic compound as anode and KVP/C as cathode, is constructed and it shows good stability for 200 cycles and a specific capacity of 80.2 mAh g −1 . This work demonstrates that vanadium‐based phosphate materials are promising high‐voltage cathode materials for ACIBs and renew the prospects for ACIBs.