Iron titanium phosphates as high-specific-capacity electrode materials for lithium ion batteries

Abstract

Two iron titanium phosphates, Fe0.5TiOPO4 and Fe0.5Ti2(PO4)(3), were prepared, and their crystal structures and electrochemical performances were compared. The electrochemical measurements of Fe0.5TiOPO4 as an anode of a lithium ion cell showed that upon the first discharge down to 0.5 V, the cell delivered a capacity of 560 mA h/g, corresponding to the insertion of 5 Li's per formula unit Fe0.5TiOPO4. Ex-situ XRD reveals a gradual evolution of the structure during cycling of the material, with lower crystallinity after the first discharge cycle. By correlating the electrochemical performances with the structural studies, new insights are achieved into the electrochemical behaviour of the Fe0.5TiOPO4 anode material, suggesting a combination of intercalation and conversion reactions. The Nasicon-type Fe0.5Ti2(PO4)(3) consists of a three-dimensional network made of corners and edges sharing [TiO6] and [FeO6] octahedra and [PO4] tetrahedra leading to the formation of trimmers [FeTi2O12]. The first discharge of lithium ion cells based on Fe0.5Ti2(PO4)(3) materials showed electrochemical activity of Ti4+/Ti3+ and Fe2+/Fe-0 couples in the 2.5-1 V region. Below this voltage, the discharge profiles are typical of phosphate systems where Li3PO4 is a product of the electrochemical reaction with lithium; moreover, the electrolyte solvent is reduced. An initial capacities as high as 1100 mA h g (1) can be obtained at deep discharge. However, there is an irreversible capacity loss in Fe0.5Ti2(PO4)(3) due to the occurrence of insulating products as Li3PO4 and a solid electrolyte interface. (C) 2013 Elsevier B. V. All rights reserved.