Effect of Strain on Electronic Structure and Polaronic Conductivity in LiFePO4

Abstract

Improving the electronic properties of active cathode materials can greatly impact the design of rechargeable batteries. We report, the influence of micro strain on the structural and electronic properties of LiFePO4 (LFP) by undertaking the combined core level spectroscopy and electrical conductivity measurements. High-resolution X-ray diffraction measurements followed by Rietveld refinement analysis reveals the increased unit cell parameters due to enhanced micro strain in the lattice structure. The effect of micro strain on the electronic structure of LFP has been probed by X-ray absorption spectroscopy (XAS) and it unravels the valence state of 3d levels in vicinity to the Fermi level, which are valunerable to the local lattice distortion. The obtained Fe L-edge and O K-edge spectra fingerprint the influence of micro strain providing valuable information on valency state of iron, crystal field and covalency character between Fe and O. The unique behaviour of structural and electronic properties of olivine LFP structure is directly linked with changes in the bonding character, which vary strongly with micro strain. We suggest that the observed lattice expansion on LFP is caused by the weaker hybridization of eg states with oxygen. The effect of micro strain on electronic properties of LFP is reflected by observing enhanced polaronic conductivity with an order of magnitude, which is highly beneficial for the unique structural advantage of improving the electrode materials.