A neutron powder diffraction study of electrochemically lithiated R-Li3+xFe2(PO4)3 for x = 1.8

Abstract

The electrochemically lithiated form of R-Li₃Fe₂(PO₄)₃, synthesised by ion exchange of fine powders of Na₃Fe₂(PO₄)₃ in a concentrated aqueous solution of LiNO₃ at a slightly elevated temperature (40 °C), has been investigated by a combination of ex situ X-ray and neutron diffraction to probe particularly the lithium-ion distribution in the structure. Li_3+xFe₂(PO₄)₃ samples were extracted from discharged electrochemical cells with a Li-metal anode; their structure was refined by the Rietveld method. Approximately 1.8 Li per formula unit can be inserted reversibly into the structure, corresponding to the reduction of almost all Fe³⁺ to Fe²⁺. Ex situ X-ray powder diffraction shows the Fe₂(PO₄)₃ framework to remain intact during lithiation. The Li(18f) site in R-Li₃Fe₂(PO₄)₃ (space group: R) is totally vacated in R-Li_3+xFe₂(PO₄)₃, 0 ≤ x ≤ 1.8, with the lithium ions moving into two new general (18f) positions, Li(1) and Li(2), with 74(4)% and 86(4)% occupation, respectively. This corresponds to the Li⁺ ions moving cooperatively from pairs of layers (alternate separations: 2.2 and 5.4 Å) for x = 0 to equally spaced layers (separation: 3.8 Å) for x = 1.8.