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

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 LixFePO4 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.