Li 0.75 Mn 1.50 Fe 1.75 (PO 4 ) 3 : First alluaudite-type iron phosphate containing only Li + as alkaline ions

A new iron phosphate Li0.75Mn1.50Fe1.75(PO4)3 has been prepared by the flux method and its structure was characterized from single crystal X-ray diffraction data. It crystallizes as an alluaudite type structure, characterized by the A(2)A(2)’A(1)A(1)’A(1)”M(1)M(2)2(PO4)3 general formula with a = 12.002(9) Å, b = 12.509(9) Å, c = 6.404(7) Å, β = 115.07(7)° in the monoclinic C2/c space group. The 3D framework consists of infinite chains of edge-sharing M(2)2O10 dimers and M(1)O6 octahedra connected by phosphate tetrahedra leading to two sets of hexagonal tunnels. The Li+ and Mn2+ ions partially occupy one of them, the other tunnel being empty. The Mössbauer spectroscopy confirmed the occurrence of only Fe3+ ions in octahedral environment. Electrochemical cycling tests in Li cells were performed using Li0.75MnII1.50FeIII1.75(PO4)3 powder as electrode material. Only a small amount of Li+ ions can be reversibility deintercalated/intercalated corresponding to a capacity as low as 30 mAh/g with a 3.2 V average voltage. Moreover, a strong polarization due to low electronic and ionic conductivities is observed. The presence of Mn2+ ions in the same tunnel as Li+ probably hinders a good Li+ ionic diffusion.