X-ray absorption spectroscopy (XAS) is a powerful technique for exploring the electronic and structural characteristics of battery materials. Historically, XAS measurements require synchrotron-based photon sources to achieve necessary sensitivity and energy control, but with breakthroughs in X-ray optics, laboratory-based XAS systems are now available on the commercial market. The Chemistry Division at the Naval Research Laboratory recently acquired an easyXAFS™ system, a laboratory-scale spectrometer that is capable of collecting XANES (X-ray Absorption Near-Edge Spectroscopy) and EXAFS (Extended X-ray Absorption Fine Structure) measurements in the 5–18 keV energy window. This energy range covers K-edge absorptions for the first-row transition metals that are of primary interest for electrochemical energy storage. In order to demonstrate the utility of such a laboratory-based XAS, we perform operando measurements on pouch-cell batteries containing the common lithium-ion battery cathode, LiFePO₄ (LFP; lithium iron phosphate), tracking Fe oxidation state and bonding environment as a function of state-of-charge. X-ray absorption spectra collected with the easyXAFS™ compare favorably to those from prior investigations of LFP cells analyzed at synchrotron sources [1]. In parallel with XAS measurements, we perform DFT (Density Functional Theory) computation of the LFP structure. Calculations of LFP show that hole-polaron formation during charging alters the local environment around the Fe ion, decreasing the Fe–O bond length. Bond contraction increases with the addition of the Hubbard "U", which also increases the valence difference (formally 2⁺/3⁺). We further refine the computational model for the LFP system by tuning the DFT input parameter “U” to correlate with experimental XAS data.

[1] C. T. Love, A. Korovina, C. J. Partridge, K. E. Swider-Lyons, M. E. Twigg, and D. E. Ramaker, J. Electrochem. Soc. 160 (2013) A3153.