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Phase Transformation and Lithiation Effect on Electronic Structure of LixFePO4: An In-Depth Study by Soft X-ray and Simulations

Liu, Xiaosong, Liu, Jun, Qiao, Ruimin, Yu, Yan, Li, Hong, Suo, Liumin, Hu, Yong-sheng, Chuang, Yi-De, Shu, Guojiun, Chou, Fangcheng, Weng, Tsu-Chien, Nordlund, Dennis, Sokaras, Dimosthenis, Wang, Yung Jui, Lin, Hsin, Barbiellini, Bernardo, Bansil, Arun, Song, Xiangyun, Liu, Zhi, Yan, Shishen, Liu, Gao, Qiao, Shan, Richardson, Thomas J., Prendergast, David, Hussain, Zahid, de Groot, Frank M. F., Yang, Wanli
Journal of the American Chemical Society 2012 v.134 no.33 pp. 13708-13715
Raman spectroscopy, X-radiation, X-ray absorption spectroscopy, crystals, electrochemistry, electrons, iron, lithium, nanoparticles, phase transition
Through soft X-ray absorption spectroscopy, hard X-ray Raman scattering, and theoretical simulations, we provide the most in-depth and systematic study of the phase transformation and (de)lithiation effect on electronic structure in LiₓFePO₄ nanoparticles and single crystals. Soft X-ray reveals directly the valence states of Fe 3d electrons in the vicinity of Fermi level, which is sensitive to the local lattice distortion, but more importantly offers detailed information on the evolution of electronic states at different electrochemical stages. The soft X-ray spectra of LiₓFePO₄ nanoparticles evolve vividly with the (de)lithiation level. The spectra fingerprint the (de)lithiation process with rich information on Li distribution, valency, spin states, and crystal field. The high-resolution spectra reveal a subtle but critical deviation from two-phase transformation in our electrochemically prepared samples. In addition, we performed both first-principles calculations and multiplet simulations of the spectra and quantitatively determined the 3d valence states that are completely redistributed through (de)lithiation. This electronic reconfiguration was further verified by the polarization-dependent spectra collected on LiFePO₄ single crystals, especially along the lithium diffusion direction. The evolution of the 3d states is overall consistent with the local lattice distortion and provides a fundamental picture of the (de)lithiation effects on electronic structure in the LiₓFePO₄ system.