Operando Combined SAXS/XRD/XAFS Measurements of Lithium Conversion Battery

Abstract

Many chemical reactions are accompanied by concerted phenomena, such as variance transition, structural changes, and particle formation. Various measurement techniques are employed to understand the whole picture of such concerted phenomena. A combination of small‐angle X‐ray scattering (SAXS), X‐ray diffraction (XRD), and X‐ray absorption fine structure (XAFS) is useful for studying concerted phenomena that occur, for example, inside rechargeable batteries. This combination can cover large lengths from 1 Å to several hundred nm. Operando measurements using this combination of methods can follow reactions during the charging and discharging of a rechargeable battery in a single experimental run. Fixed‐exit optics enable irradiation at the same point in a wide energy range. By employing 2D detectors for XRD and SAXS and using a quick XAFS technique, one can make the interval of each measurement sufficiently short to track various phenomena during charging and discharging. Here, an application of the system for alternating SAXS/XRD/XAFS measurements constructed in BL28XU, SPring‐8, Japan to the study of rechargeable batteries, is presented.

Temperature Driven Phase Evolution of FeF3 as Active Material in Lithium-Ion Batteries

“…32 This partial conversion can be due to the cutoff voltage for the conversion mechanism (i.e., 1.2 V vs Li + /Li in this study) as suggested previously. 32 However, at low potentials for conversion active materials, Simon et al showed that irreversible electrolyte reduction mechanisms can occur, affecting the reversibility of the cell assembly. 40 An in-depth comprehension of the different mechanisms occurring in the cell is thus necessary in order to determine the appropriate cutoff voltage of the conversion materials.…”

“…The reduction path followed at elevated temperatures involved the formation of FeF 2 type structure, as calculated by Doe et al 30 However, the oxidation at room temperature forming the Li x FeF 3 structure in the R3c space group differs from both reported mechanisms but has been recently observed by Gray et al and Takabayashi et al when working at room temperature with FeF 3 . 31,32 The oxidation at 80 °C formed mainly FeF 2 , as evidenced by XRD and XAS, in consistency with the mechanism proposed by Badway et al and later calculated by Doe et al 29,30 This mechanism path indicate that after a first reduction of FeF 3 , the active material is mainly reproducing a FeF 2 type (i.e., two electrons exchanged) mechanism for the following cycles of oxidation and reduction at elevated temperature, whereas at room temperature the mechanism path involves a higher number of electrons exchanged. However, it is interesting to notice that despite a lower number of exchanged electrons, the FeF 3 at higher temperature exhibits better performance than at room temperature (Figure 2).…”

“…This study links the difference in electrochemical mechanism to the particle size of FeF 3 ; however, the recent results with SPEs at temperatures above 60 °C could indicate that the two lithiation mechanisms could be related to the temperature rather than the particle size of FeF 3 . More recently, Takabayashi et al investigated the reduction and the oxidation electrochemical mechanism of FeF 3 at room temperature using complementary three X-ray operando techniques, i.e., small-angle X-ray scattering (SAXS), X-ray diffraction (XRD), and X-ray absorption fine structure (XAFS) . Using simultaneously these three techniques allows to comprehend the chemical, structure, and morphological variations occurring during both the Fe(III)/Fe(II) and Fe(II)/Fe(0) electrochemical reactions.…”

“…27 More recently, Takabayashi et al investigated the reduction and the oxidation electrochemical mechanism of FeF 3 at room temperature using complementary three X-ray operando techniques, i.e., small-angle X-ray scattering (SAXS), X-ray diffraction (XRD), and X-ray absorption fine structure (XAFS). 32 Using simultaneously these three techniques allows to comprehend the chemical, structure, and morphological variations occurring during both the Fe(III)/Fe(II) and Fe(II)/Fe(0) electrochemical reactions. They showed that during the first stage of the lithiation of FeF 3 , a topotactic insertion phase of Li x FeF 3 in the R3c space group is formed, in agreement with the predicted mechanism of Badway et al and the findings of Grey et al 31 They found that a second stage of lithiation involved a phase transition to a P42/mnm type structure, in agreement with the mechanism calculated by Doe et al and observed by Grey et al at elevated temperature.…”

“…30 The operando SAXS measurements allowed Takabayashi et al to monitor the real-time evolution of the Fe and Li-rich domains during the discharge, evidencing the conversion mechanism and the formation of Fe(0) and LiF. 32 They observed that the consecutive oxidation of the Fe + 3 LiF electrode leads to the formation of a LiFe 2 F 6 type structure in the space group P42/mnm, in agreement with the previously reported findings of Grey et al 31 These recent publications underline the complex mechanisms involved during the reduction and oxidation of FeF 3 and demonstrate the need to conduct operando measurements in order to understand these phase transitions, particularly for the transient species formed.…”