Lithium insertion into rhombohedral Li3Fe2(PO4)3

The lithium insertion process has been studied in rhombohedral Li3Fe2(PO4)3 (NASICON-type structure) by electrochemical and Mössbauer spectroscopic methods. The form of the discharge curve and the effective discharge capacity is found to depend on the mode of cathode preparation: two plateaus (one clear at ∼2.80 V and one less distinct at ∼2.65 V vs. Li/Li+), corresponding to ca. 1.5–1.6 inserted lithium ions during the first cycle, are seen after more extreme grinding; milder treatment gave only the 2.8 V plateau and ca. 1.1 inserted lithium ions. Mössbauer spectra for the more extensively ground material show the Fe environments in R-Li3Fe2(PO4)3 to be highly symmetric; only a very narrow doublet with small quadrupolar splitting is observed, and the two crystallographically independent Fe-atoms cannot be distinguished. As lithium insertion proceeds, two doublets (average intensity ratio 1.5:1) appear, which can be assigned to two Fe2+ sites. The average intensity ratio of 1.5:1 suggests that the extra lithium ions occupy sites closer to one of the Fe-atoms.