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

Abstract

Here we report the observation of electron delocalization in nano-dimension xLiFePO₄:(1 − x)FePO₄ (x = 0.5) using high temperature, static, ³¹P solid state NMR. The ³¹P paramagnetic shift in this material shows extreme sensitivity to the oxidation state of the Fe center. At room temperature two distinct ³¹P resonances arising from FePO₄ and LiFePO₄ are observed at 5800 ppm and 3800 ppm, respectively. At temperatures near 400 °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. ⁷Li MAS NMR spectra of nanometre sized xLiFePO₄:(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 ⁷Li MAS spectrum of LiMnPO₄, which despite being iso-structural with LiFePO₄ yields a remarkably different ⁷Li MAS spectrum due to the different electronic states of the paramagnetic centers. For LiMnPO₄ the higher ⁷Li MAS paramagnetic shift (65 ppm) and narrowed isotropic resonance (FWHM ≈ 500 Hz) is attributed to an additional unpaired electron in the t_2g orbital as compared to LiFePO₄ which has δ_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 ⁷Li MAS spectrum of xLiFePO₄:(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.