Phase analysis of (Li)FePO₄ by selected area electron diffraction and integrated differential phase contrast imaging

Abstract text (incl. figure legends and references)

Lithium iron phosphate (LiFePO₄) is a well-studied compound with a lot of promise as cathode material in rechargeable batteries. Due to its low cost, low toxicity, safety and the abundance of iron LFP is considered a very attractive energy storage option for the automotive industry.

LiFePO₄ has an orthorhombic crystal structure with Pnma space group [1]. During the discharge process lithium intercalates from a graphite anode into the FePO₄ cathode, where it is stored in between FeO₆ octahedra and PO₄ tetrahedra, thus slightly changing the lattice vector length of the unit cell while maintaining the same crystal structure.

Our aim is to better understand the lithium deintercalation process in (Li)FePO₄ battery cells on atomic and macroscopic scale. Fully delithiated, fully lithiated and partially lithiated cells are prepared using chemical- and electrochemical delithiation as well as bio templating.

We use Selected Area Electron Diffraction (SAED) and integrated Differential Phase Contrast imaging (iDPC) in the TEM in order to differentiate between lithiated and (partially) delithiated particles. FIB lamellas are prepared from electrochemically delithiated cells for this purpose.

We also aim to compare the results from our findings in the TEM with Raman microscopy measurements. Preliminary Raman experiments on bio-templated (Li)FePO4 already showed that LiFePO₄ and FePO4 phases are differentiable with their respective Raman-shift.

With SAED measurements we successfully managed to differentiate between LiFePO₄ and FePO₄ phases as well as partially delithiated phases for single particles as seen in figure 1. High-Resolution STEM as well as iDPC imaging have confirmed the feasibility of SAED for detection of lithium content. The lattice spacings obtained by HR-STEM FFT analysis were similar to the lattice spacings obtained by SAED. Using iDPC imaging we were able to directly show the presence of lithium in a partially delithiated particle as seen in figure 3.

We conclude that SAED analysis is feasible for differentiation between lithiated and (partially) delithiated states in Li_xFePO₄ as confirmed by HR-STEM FFT analysis and iDPC imaging. Raman microscopy may provide further insight on the delithiation process on a macroscopic scale in future work.

Figure 1: Diffraction pattern of a LiFePO₄ particle in 011 direction. The lattice spacing of the 001 plane indicates partial delithiation after comparison with simulated powder diffraction data from LiFePO4 and Li_0.6FePO₄.

Figure 2: Theoretical model of LiFePO₄ in 011 direction. The model shows Li atoms in green, Fe in orange, O in red and P in grey.

Figure 3: Integrated Differential Phase Contrast image of the same particle seen in figure 1. All corresponding columns in the image show clear contrast at the theoretical Li positions which implies Li presence, thus confirming the diffraction results. See figure 2 for better reference with the theoretical model.

[1] V.S.L. Satyavani, A. Srinivas Kumar, P.S.V. Subba Rao, et. al., Engineering Science and Technology, an International Journal, 19 (1), 178-188 (2016)