Local structure in the Li-ion battery cathode material Lix(MnyFe1−y)PO4 for 0 < x ≤ 1 and y = 0.0, 0.5 and 1.0

Infrared and Raman spectroscopy have been used to investigate the Li-ion battery cathode materials Lix(MnyFe1−y)PO4 for 0 < x ≤ 1 and y = 0.0, 0.5 and 1.0. In the Raman spectrum of Li(Mn0.5Fe0.5)PO4, the frequency of the PO_4^(3-)symmetric stretching mode is very similar to that found in LiFePO4 and LiMnPO4, suggesting that the P–O bond lengths are similar across the entire Li(MnyFe1−y)PO4 solid-solution series. The more localized ν1 and ν3 bands (symmetric and antisymmetric PO_4^(3-) stretching modes, respectively) in the infrared spectra exhibit two-mode behavior for all three Li(MnyFe1−y)PO4 compositions studied, whereas the PO_4^(3-) antisymmetric bending modes (ν4) and the Li+ cage mode frequencies vary with Mn2+ concentration. The vibrational modes in Lix(Mn0.5Fe0.5)PO4 are sensitive to lithium content (x) and are, thus, used to investigate changes in the (Mn0.5Fe0.5)PO4 framework under cycling. The lack of spectral changes during the Fe2+/Fe3+ redox couple (3.7 V versus Li+/Li) suggests that the PO_4^(3-) anions have similar local environments in Li(Mn0.5(II)Fe0.5(II))PO4 and Li0.5(Mn0.5(II)Fe0.5(III))PO4. Two-phase behavior is confirmed during the Mn2+/Mn3+ redox couple (4.2 V), and the infrared spectrum for (Mn0.5(III)Fe0.5(III))PO4 is similar to that of FePO4 since both phases contain PO_4^(3-) anions coordinated only to trivalent transition metal ions. However, the PO_4^(3-) anions are much more distorted in (Mn0.5(III)Fe0.5(III))PO4 compared to FePO4, probably as a result of deformation of the MnO6 octahedra induced by Jahn–Teller active Mn3+ ions in the (Mn0.5(III)Fe0.5(III))PO4 structure. Raman spectra suggest that the carbon layer coating the Li(Mn0.5Fe0.5)PO4 particles (to improve electronic contact between particles) is predominantly composed of sp2-hybridized carbon atoms.