The Spin-Polarized Electronic Structure of LiFePO₄ and FePO₄ Evidenced by in-Lab XPS

Abstract

LiFePO 4 has an interesting spin-polarized electronic structure showing a (3dv) 5 (3dV) 1 electron configuration of the Fe 2+ ion. In this work, we have experimentally evidenced the valence electronic structure of LiFePO 4 and of its delithiated compound FePO 4 by X-ray photoelectron spectroscopy (XPS), which allows a visualization of the occupied densities of states (DOS) in the valence band. XPS valence spectra were compared with the DOS obtained from DFT calculations by considering GGA and GGA + U approaches. Thanks to electrochemical extraction/insertion of Li + ions in LiFePO 4 /FePO 4 , it was possible to display the Fe 3d spindown electron of LiFePO 4 , which is not present in the valence spectrum of FePO 4 . We show that the study of XPS valence spectra is an efficient way to access the lithium insertion rate in Li x FePO 4 positive electrode materials for lithium-ion batteries. Besides the contribution to the Li-ion battery field, this paper is also a rare example of experimental evidence of a spin-resolved electronic structure from in-lab XPS experiments.

Study of C-coated LiFe0.33Mn0.67PO4 as positive electrode material for Li-ion batteries

“…Details about this spectral shape can be found elsewhere [24]. The Fe 2p spectrum of C-LiFePO 4 reference material, showing a Fe 2p 3/2 main peak at 710 eV (peak A) and a shake-up satellite at 715 eV, is in good agreement with previous works and is characteristic of Fe 2 þ cations in the olivine structure [25,26,27].…”

“…The same evolution can be also clearly observed for the Fe 2p 1/2 component (peaks A 0 and C 0 ). A this stage it is possible to estimate the Fe 3 þ /Fe 2 þ ratio at the extreme surface of the material at about 35/65 by fitting the spectrum with a linear combination of the Fe 2p spectra of C-LiFePO 4 and C-FePO 4 reference samples (C-FePO 4 obtained from complete delithiation of C-LiFePO 4 [27]). The Mn 2p spectrum shows no significant change with respect to the starting C-LiFe 0.33 Mn 0.67 PO 4 electrode.…”

Optimization of Electrochemical Performance of LiFePO4/C by Indium Doping and High Temperature Annealing

“…The Fe elemental XPS spectra of In-LFP-600 and In-LFP-700 samples are shown in Figure 3. All the spectra were fitted with three peaks, two at 710 and 714.5 eV are due to Fe 2+ ions in LiFePO 4 and the third one at 712 eV due to Fe 3+ originate mainly from FePO 4 and/or Fe 2 P in agreement with the literature values [29,30]. The amount of ferric iron in In-LFP-600 was determined to be lower (~10%) compared to In-LFP-700 (~13%) sample, which is consistent with the Mössbauer spectroscopy measurements as described in Section 2.5.…”

How Mössbauer spectroscopy can improve Li-ion batteries

“…Compared to LiFePO 4 , the value of δ decreases because the extraction of one lithium is accompanied with the decrease of one Fe3d↓ electron per Fe, changing high spin Fe 2+ (in LiFePO 4 ) into high spin Fe 3+ (in FePO 4 ). This affects the electronic structure near the Fermi level as shown by XPS and DFT calculations [25]. The observed decrease of is also related to the decrease of one Fe 3d electron per Fe that strongly influences the Fe 3d electron density shape.…”