Electrochemical Strain Microscopy of Nanostructured Lithium Iron Phosphate

Lithium iron phosphate (LFP) has been explored as a promising alternative to the most commonly used lithium cobalt oxide as a cathode material for lithium ion batteries. Due to its olivine structure, lithium ions within LFP travel along one-dimensional channels, and the lithiation mechanism in LFP occurs through two-phase reactions that are still a major area of study. In this paper, we use electrochemical strain microscopy (ESM) in band excitation mode to probe lithium ion motion within thin films of LFP.  ESM is a nanoscale probing technique in which an AC bias is applied to the surface of the electrolyte material, locally inducing lithium ion motion and causing strain changes, which can then be detected by the tip of an scanning probe microscope probe. Using band excitation techniques, strain amplitudes as well as energy losses can be evaluated for very rough samples of LFP. Furthermore, lithium ion motion was also studied in terms of its second harmonic response to AC bias, which can be compared to its first harmonic response within the same topographic area. These experiments can lead to increased fundamental understanding of lithium iron phosphate and other cathode materials of similar structures.