Chromium substitution in ion exchanged Li3Fe2(PO4)3 and the effects on the electrochemical behavior as cathodes for lithium batteries

A new family of cathode materials for lithium cells with nominal stoichiometry Li3Fe2−xCrx(PO4)3 (0 ≤ x ≤ 1), was successfully prepared by ion exchange reactions of Na3Fe2−xCrx(PO4)3. The chemical exchange process leads to the structural transition from the monoclinic to a rhombohedral phase with an open framework which facilitates lithium diffusion. Chromium substitution was evidenced by a decrease in cell parameters in both sodium and lithium phases. 57Fe Mössbauer spectra revealed a decrease in the isomer shift on increasing Cr content. The partial substitution by low Cr contents improved the electrochemical performance of Li3Fe2−xCrx(PO4)3. Thus, Li3Fe1.8Cr0.2(PO4) reversibly inserted 1.7 Li ions per formula unit, delivering a specific capacity of 105 mAh/g after the first discharge. The low electrode impedance of Cr substituted compounds can be responsible for the improvement of the electrochemical behavior.