Local structure in the Li-ion battery cathode material Li x(Mn yFe 1− y)PO 4 for 0 < x ≤ 1 and y = 0.0, 0.5 and 1.0

Abstract

Infrared and Raman spectroscopy have been used to investigate the Li-ion battery cathode materials Li x (Mn y Fe 1− y )PO 4 for 0 < x ≤ 1 and y = 0.0, 0.5 and 1.0. In the Raman spectrum of Li(Mn 0.5Fe 0.5)PO 4, the frequency of the P O 4 3 − symmetric stretching mode is very similar to that found in LiFePO 4 and LiMnPO 4, suggesting that the P–O bond lengths are similar across the entire Li(Mn y Fe 1− y )PO 4 solid-solution series. The more localized ν 1 and ν 3 bands (symmetric and antisymmetric P O 4 3 − stretching modes, respectively) in the infrared spectra exhibit two-mode behavior for all three Li(Mn y Fe 1− y )PO 4 compositions studied, whereas the P O 4 3 − antisymmetric bending modes (ν 4) and the Li + cage mode frequencies vary with Mn 2+ concentration. The vibrational modes in Li x (Mn 0.5Fe 0.5)PO 4 are sensitive to lithium content ( x) and are, thus, used to investigate changes in the (Mn 0.5Fe 0.5)PO 4 framework under cycling. The lack of spectral changes during the Fe 2+/Fe 3+ redox couple (3.7 V versus Li +/Li) suggests that the P O 4 3 − anions have similar local environments in Li(Mn 0.5 (II)Fe 0.5 (II))PO 4 and Li 0.5(Mn 0.5 (II)Fe 0.5 (III))PO 4. Two-phase behavior is confirmed during the Mn 2+/Mn 3+ redox couple (4.2 V), and the infrared spectrum for (Mn 0.5 (III)Fe 0.5 (III))PO 4 is similar to that of FePO 4 since both phases contain P O 4 3 − anions coordinated only to trivalent transition metal ions. However, the P O 4 3 − anions are much more distorted in (Mn 0.5 (III)Fe 0.5 (III))PO 4 compared to FePO 4, probably as a result of deformation of the MnO 6 octahedra induced by Jahn–Teller active Mn 3+ ions in the (Mn 0.5 (III)Fe 0.5 (III))PO 4 structure. Raman spectra suggest that the carbon layer coating the Li(Mn 0.5Fe 0.5)PO 4 particles (to improve electronic contact between particles) is predominantly composed of sp 2-hybridized carbon atoms.