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

摘要

Two iron titanium phosphates, Fe_(0.5)TiOPO_4 and Fe_(0.5)Ti_2(PO_4)_3, were prepared, and their crystal structures and electrochemical performances were compared. The electrochemical measurements of Fe_(0.5)TiOPO_4 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 Fe_(0.5)TiOPO_4. 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 stud ies, new insights are achieved into the electrochemical behaviour of the Fe_(0.5)TiOPO_4 anode material, sug gesting a combination of intercalation and conversion reactions. The Nasicon-type Fe_(0.5)Ti_2(PO_4)_3 consists of a three-dimensional network made of corners and edges sharing [TiO_6] and [FeO_6] octahedra and [PO_4] tetrahedra leading to the formation of trimmers [FeTi_2O_(12)]. The first discharge of lithium ion cells based on Fe_(0.5)Ti_2(PO_4)_3 materials showed electrochemical activity of Ti~(4+)/Ti~(3+) and Fe~(2+)/Fe~° couples in the 2.5-1 V region. Below this voltage, the discharge profiles are typical of phosphate systems where U3PO4 is a product of the electrochemical reaction with lithium; moreover, the electrolyte solvent is reduced. An initial capacities as high as 1100 mAhg~(-1) can be obtained at deep discharge. However, there is an irreversible capacity loss in Fe_(0.5)Ti_2(PO_4)_3 due to the occurrence of insulating products as Li_3PO_4 and a solid electrolyte interface.