title: On the Mechanism of the Improved Operation Voltage of Rhombohedral Nickel Hexacyanoferrate as Cathodes for Sodium-Ion Batteries.
abstract: We reported a rhombohedral Na-rich nickel hexacyanoferrate (r-NiHCF) with high discharge voltage, which also possesses long cycle stability and excellent rate capability when serving as the cathode material of Na-ion batteries. First-principles calculations suggest that the high working voltage of r-NiHCF is correlated to the asymmetric residence of Na+ions in the rhombohedral framework in parallel with the low charge density at the Fe2+ions. In both aqueous and ether-based electrolytes, r-NiHCF exhibits higher voltage than that of cubic NiHCF. Rate and cycle experiments indicate that r-NiHCF delivers a specific capacity of 66.8 mAh g-1at the current density of 80 mA g-1, which is approximate to the theoretical capacity of r-NiHCF. A capacity retention of 96% can be achieved after 200 cycles. The excellent stability of r-NiHCF can be assigned to the absence of rhombohedral-cubic phase transition and negligible volume variation during electrochemical redox, as proven by the ex situ XRD patterns at different depths of charge/discharge and the DFT calculations, respectively.
authors.affiliation: Sustainable Energy Laboratory, Faculty of Materials Science and Chemistry, China University of Geosciences , Wuhan 430074, Hubei, P. R. China.
authors.forename: Zhuan
authors.initials: Z
authors.lastname: Ji
authors.affiliation: Sustainable Energy Laboratory, Faculty of Materials Science and Chemistry, China University of Geosciences , Wuhan 430074, Hubei, P. R. China.
authors.forename: Bo
authors.initials: B
authors.lastname: Han
authors.affiliation: Sustainable Energy Laboratory, Faculty of Materials Science and Chemistry, China University of Geosciences , Wuhan 430074, Hubei, P. R. China.
authors.forename: Haitao
authors.initials: H
authors.lastname: Liang
authors.affiliation: Sustainable Energy Laboratory, Faculty of Materials Science and Chemistry, China University of Geosciences , Wuhan 430074, Hubei, P. R. China.
authors.forename: Chenggang
authors.initials: C
authors.lastname: Zhou
authors.affiliation: Sustainable Energy Laboratory, Faculty of Materials Science and Chemistry, China University of Geosciences , Wuhan 430074, Hubei, P. R. China.
authors.forename: Qiang
authors.initials: Q
authors.lastname: Gao
authors.affiliation: Sustainable Energy Laboratory, Faculty of Materials Science and Chemistry, China University of Geosciences , Wuhan 430074, Hubei, P. R. China.
authors.forename: Kaisheng
authors.initials: K
authors.lastname: Xia
authors.affiliation: Sustainable Energy Laboratory, Faculty of Materials Science and Chemistry, China University of Geosciences , Wuhan 430074, Hubei, P. R. China.
authors.forename: Jinping
authors.initials: J
authors.lastname: Wu
keywords: Na-ion batteries; cathode material; density functional theory study; high discharge voltage; mechanism; rhombohedral nickel hexacyanoferrate