Near-field Nanoscopy and Spectroscopy of Phase Coexistence in LiFePO$_4$ Electrode Microcrystals

Due to instrumental limitations, the microscopic description of lithiation and delithiation processes in low-cost LiFePO$_4$ electrodes has remained uncertain and subject to controversy. Using infrared near-field imaging, we present evidence for a novel coexistence of phases within single LiFePO$_4$ microcrystals. First-principles calculations of the phonon response of lithiated and delithiated end-phases are compared with broadband nano-FTIR (Fourier transform infrared) spectroscopy data to reveal the mid-infrared vibrational signature of lithiation. By resolving this signature at the nano-scale, we observe a propagation of phase boundaries within these crystals over the course of chemical delithiation. In addition, by comparing theoretical modeling with spatially resolved nano-FTIR spectra measured across a single crystal at partial delithiation, we assemble a tomographic view of phases distributed hundreds nanometers beneath the crystal surface. These experiments set the stage for quantitative nano-spectroscopy of new composite electrode materials, assisting in the rational design of next-generation electrical energy storage systems.