Length‐Scale‐Dependent Phase Transformation of LiFePO₄: An In situ and Operando Study Using Micro‐Raman Spectroscopy and XRD

Abstract

The charge and discharge of lithium ion batteries are often accompanied by electrochemically driven phase-transformation processes. In this work, two in situ and operando methods, that is, micro-Raman spectroscopy and X-ray diffraction (XRD), have been combined to study the phase-transformation process in LiFePO4 at two distinct length scales, namely, particle-level scale (∼1 μm) and macroscopic scale (∼several cm). In situ Raman studies revealed a discrete mode of phase transformation at the particle level. Besides, the preferred electrochemical transport network, particularly the carbon content, was found to govern the sequence of phase transformation among particles. In contrast, at the macroscopic level, studies conducted at four different discharge rates showed a continuous but delayed phase transformation. These findings uncovered the intricate phase transformation in LiFePO4 and potentially offer valuable insights into optimizing the length-scale-dependent properties of battery materials.

Application of Operando X-ray Diffraction and Raman Spectroscopies in Elucidating the Behavior of Cathode in Lithium-Ion Batteries

“…Several models, such as Shrinking core model (Padhi et al, 1997), Domino cascade model (Delmas et al, 2008) describe the phase transformation between LiFePO 4 and FePO 4 , but the mechanism is still in debate. As a result, several operando techniques were applied to understand the process (Perea et al, 2012;Orikasa et al, 2013;Ouvrard et al, 2013;Dong et al, 2017), especially using operando XRD (Shin et al, 2008;Roberts et al, 2014) and operando Raman Siddique et al, 2015). The LFP↔FP transition is generally considered as a two-phase reaction (Chen et al, 2006;Laffont et al, 2006;Zhu Y. et al, 2013), also confirmed by XRD measurements conducted on slow cycled cells (Andersson et al, 2000;Shin et al, 2011;Siddique et al, 2015).…”

“…(B2) diffraction patterns from first two cycles stacked against the voltage profile; black vertical lines mark the positions of LFP peaks at the start of reaction; red vertical lines mark the position of FP peaks formed during the first cycle; with permission from AAAS. (C) Raman spectra collected on LFP electrode surface at a cycling rate of 0.3C (Siddique et al, 2015); (C1) carbon-rich spot A during charge; (C2) spot A during discharge; (C3) carbon-poor spot B during charge; (C4) spot B during discharge; (C5) corresponding electrochemical cell performance curve with vertical lines indicating the moment when the Raman spectra were taken; with permission from Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim.…”

Investigation of the Phase Transition Mechanism in LiFePO4 Cathode Using In Situ Raman Spectroscopy and 2D Correlation Spectroscopy during Initial Cycle

“…in real time. In the past few years, many researchers have tried to monitor in real time reactions inside batteries during electrochemical performance by using XRD [1,2,3,4], XAS [5,6,7], Raman [8,9,10,11,12,13,14,15,16,17], NMR [18], and so forth [19,20]. However, the studies for electrochemical reactions during the initial cycle is still limited.…”

“…Recently, C. Delmas et al reported another phase transition mechanism, which is named the ‘domino-cascade’ model [31]. They assume that phase transformations at particle-level are fast, and small particles appear either fully charged (FePO 4 phase) or discharged (LiFePO 4 phase) due to a very rapid two-phase front [4,31]. The phase transition mechanism of LiFePO 4 has been also investigated through in situ Raman spectroscopy [4,13].…”

“…They assume that phase transformations at particle-level are fast, and small particles appear either fully charged (FePO 4 phase) or discharged (LiFePO 4 phase) due to a very rapid two-phase front [4,31]. The phase transition mechanism of LiFePO 4 has been also investigated through in situ Raman spectroscopy [4,13]. Many models of phase transformation in LiFePO 4 were reported; however the understanding is still limited.…”