Phase Analysis of (Li)FePO4 by Selected Area Electron Diffraction in Transmission Electron Microscopy

Lithium iron phosphate (LiFePO4) is a well-studied compound with a lot of promise as cathodematerial in rechargeable batteries. Due to its low cost, low toxicity, safety and the abundance ofiron 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 dischargeprocess lithium intercalates from a graphite anode into the FePO4 cathode, where it is stored inbetween FeO6 octahedra and PO4 tetrahedra, thus slightly changing the lattice vector length of theunit cell while maintaining the same crystal structure as seen in figure 1.To better understand the lithium deintercalation process various studies were performed withmethods such as x-ray diffraction [2] and precession diffraction [3] to identify charged anddischarged (L)FP particles by measuring lattice spacings.This work shows the identification process of (Li)FePO4 particles via selected area electrondiffraction (SAED) with comparison of theoretical calculations of respective crystal models. SAEDpatterns have been recorded for numerous particles with size of approximately 200 nm in eitherlithiated (LiFePO4) or delithiated (FePO4) samples with results matching expectations. Throughrigorous experiments the presented methodology has been deemed reliable and applied tosamples that are either fully lithiated (LiFePO4), partially delithiated (LixFePO4), or fully delithiated(FePO4). A comparison of chemically and electrochemically delithiated samples is made with bothSAED as well as Raman spectroscopy.