In Situ Studies of Materials and Dynamics for Li Ions in Phosphate and Other Electrodes Using X-Ray Techniques in SPring-8
We have recently completed a beam line for battery researches at BL-28XU in SPrign-8, founded by NEDO (New Energy and Industrial Technology Development Organization) RISING (Research & Development Initiative for Scientific Innovation of New Generation Batteries) Project. In the present presentation, I will show our recent research achievements using this beam-line. A structural changes of electrode materials during lithiation/delithiation in Li ion batteries (LIBs) are important to analyze and understand the performance of LIBs.
LiFePO4 is a well-known and promising active material for LIBs, which is characterized by a two-phase (LiFePO4, FePO4) reaction during charge and discharge. In this material, strain due to a lattice mismatch between the two phases in the phase transformation greatly affects the battery property. In situ XRD and XAFS techniques were simultaneously carried out at the same “time” and “region” on a composite electrode, which revealed the kinetics of the LiFePO4 electrode during the battery reaction without being disturbed by the reaction inhomogeneity of the electrode. In this study, we observed a delay of structural change after a valence change of Fe ions in LiFePO4 during delithiation and discussed it from the view point of the strain energy.
Furthermore, the diffraction anomalous fine structure (DAFS) method, which is a spectroscopic analysis established by coupling XRD with XAFS, was conducted to analyze electrode materials for LIBs. In DAFS measurements, diffraction intensities are measured as a function of X-ray photon energy. Consequently, XAFS-like spectra can be obtained at each phase and site. In the past, DAFS technique was believed to be a time-consuming measurement (e.g. 1 day) and its analysis, especially the correction of the absorption term, was complicated.
Thus, its applications were limited such as single crystals or thin films. We have developed the relatively fast DAFS measurement technique and proposed a simple analytical method without any iteration process. We consider DAFS is extremely useful in the structural analyses of oxides in positive electrodes because of its site-selectivity and/or phase-selectivity as well as the element-selectivity.