Soft chemical routes to electrochemically active iron phosphates


New iron phosphates with related structures have been synthesized using hydrothermal and ion-exchange routes, and their electrochemical properties were investigated. First, NaFe(HPO 4 ) 2 was synthesized employing a hydrothermal route and its structure was determined from single-crystal X-ray diffraction data. Subsequent Na + and partial proton ion exchange with Li + ion produced a known phase, Li 2 Fe(H 0.5 PO 4 ) 2 , and complete deprotonation of Li 2 Fe(H 0.5 PO 4 ) 2 with Li + by employing a solid-state ion-exchange route produced the new phase Li 3 Fe(PO 4 ) 2 . The structure of the latter was solved from synchrotron powder X-ray data by employing ab initio methods. All of these phases are highly crystalline, built up of similar connectivities between FeO 6 octahedra and PO 4 tetrahedral units. Magnetic susceptibility measurements and room-temperature 57 Fe Mössbauer spectroscopic studies confirm the 3+ oxidation state of the compounds and their antiferromagnetic ordering with Li 2 Fe(H 0.5 PO 4 ) 2 showing some interesting metamagnetic behavior. The compounds are stable up to 400 °C and undergo facile electrochemical lithium/sodium insertion through the reduction of Fe 3+ to Fe 2+ . Galvanostatic charge-discharge studies indicate that up to 0.6 lithium ion and 0.5 sodium ion per formula unit can be inserted at average voltages of 3.0 and 2.75 V for lithium and sodium ion batteries, respectively, for NaFe(HPO 4 ) 2 . The partially Li ion exchanged compound Li 2 Fe(H 0.5 PO 4 ) 2 showed better cycle life and experimentally achievable capacities up to 0.9 Li insertion with strong dependence on particle size. The electrochemical Li insertion in Li 3 Fe(PO 4 ) 2 was also investigated. The electrochemistry of these three related phases were compared with each other, and their mechanism of Li insertion was investigated by ex situ PXRD. ©


Physics, Astronomy, and Materials Science

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Inorganic Chemistry