Ultra-stable Ti2O(PO4)2(H2O) as a viable new Ca2+ storage electrode material for calcium-ion batteries

Abstract

The practical application of calcium ion batteries (CIBs) suffers from a lack of reliable electrode materials that have a long cycle-life and less severe hysteric and capacitive voltage-behavior. Here, we for the first time describe Ti2O(PO4)2(H2O) as a new Ca2+ insertion electrode material for CIBs. Hydrothermally synthesized Ti2O(PO4)2(H2O) can reversibly store ca. 0.51 Ca2+ (ca. 85 mAh g−1) at ca. 2.6 V vs. Ca/Ca2+ at room temperature. This new material displays an unprecedented long cyclability by retaining ca. 95% of the initial capacity after 1500 charge/discharge (C/D) cycles. The structural and compositional characterizations firmly substantiate reversible Ca2+ insertion associated with Ti4+/Ti3+ redox. In-situ X-ray diffraction (XRD) studies reveal that the electrochemical insertion/extraction of Ca2+ ions in Ti2O(PO4)2(H2O) accompanies a minimal dimensional change with no transformation in the crystallographic structure, which is believed to be responsible for the exceptional stability. Bond valence site energy (BVSE) and density functional theory (DFT) calculations also demonstrate that, in contrast to reversible Li+ insertion at ca. 1.7 V in isostructural M0.5TiO(PO4) (M = Cu2+, Mg2+, Co2+, Ni2+, or Fe2+), the significant potential upshift to 2.6 V in Ti2O(PO4)2(H2O) is related to a unique Ca2+ location and migration path, which is ascribed to different orientations of PO4 tetrahedra and TiO6 octahedra. Finally, we construct a CIB cell with a configuration of K metal ‖ Ti2O(PO4)2(H2O) by employing K+-containing hybrid electrolytes, and conceptually show the applicability of Ti2O(PO4)2(H2O) as a viable cathode for CIBs.