Influence of particle size on solid solution formation and phase interfaces in Li0.5FePO4 revealed by P-31 and Li-7 solid state NMR spectroscopy

Here we report the observation of electron delocalization in nano-dimension xLiFePO(4):(1 - x)FePO4 (x = 0.5) using high temperature, static, P-31 solid state NMR. The P-31 paramagnetic shift in this material shows extreme sensitivity to the oxidation state of the Fe center. At room temperature two distinct P-31 resonances arising from FePO4 and LiFePO4 are observed at 5800 ppm and 3800 ppm, respectively. At temperatures near 400 degrees C these resonances coalesce into a single narrowed peak centered around 3200 ppm caused by the averaging of the electronic environments at the phosphate centers, resulting from the delocalization of the electrons among the iron centers. Li-7 MAS NMR spectra of nanometre sized xLiFePO(4):(1 x)FePO4 (x - 0.5) particles at ambient temperature reveal evidence of Li residing at the phase interface between the LiFePO4 and FePO4 domains. Moreover, a new broad resonance is resolved at 65 ppm, and is attributed to Li adjacent to the anti-site Fe defect. This information is considered in light of the Li-7 MAS spectrum of LiMnPO4, which despite being iso-structural with LiFePO4 yields a remarkably different Li-7 MAS spectrum due to the different electronic states of the paramagnetic centers. For LiMnPO4 the higher Li-7 MAS paramagnetic shift (65 ppm) and narrowed isotropic resonance (FWHM approximate to 500 Hz) is attributed to an additional unpaired electron in the t(2g) orbital as compared to LiFePO4 which has delta(iso) = -11 ppm and a FWHM = 9500 Hz. Only the delithiated phase FePO4 is iso-electronic and iso-structural with LiMnPO4. This similarity is readily observed in the Li-7 MAS spectrum of xLiFePO(4):(1 - x)FePO4 (x = 0.5) where Li sitting near Fe in the 3+ oxidation state takes on spectral features reminiscent of LiMnPO4. Overall, these spectral features allow for better understanding of the chemical and electrochemical (de)lithiation mechanisms of LiFePO4 and the Li-environments generated upon cycling.