Effect of strain on the electronic structure and polaronic conductivity of LiFePO4†

Abstract

Improving the electronic properties of active cathode materials can significantly impact the design of rechargeable batteries. In this study, we investigated the influence of micro-strain on the structural and electronic properties of LiFePO₄ (LFP) by performing combined core-level spectroscopy analysis and electrical conductivity measurements. High-resolution X-ray diffraction measurements, followed by Rietveld refinement analysis, revealed an increase in unit cell parameters due to the enhanced micro-strain in the lattice structure. ⁵⁷Fe Mössbauer spectroscopy disclosed the presence of Fe²⁺ and Fe³⁺ in distorted octahedral environments, and their relative concentrations provided a comprehensive understanding of the electronic structure and its relationship with micro-strain in the LFP samples. The effect of micro-strain on the electronic structure of the LFP samples was investigated using X-ray absorption spectroscopy (XAS). The analysis revealed the valence state of the 3d levels in the vicinity of the Fermi level, which was sensitive to local lattice distortions. The obtained Fe L-edge and O K-edge spectral fingerprints demonstrated the influence of micro-strain, providing valuable insights into the valence state of iron, crystal field and covalent character between Fe and O. The unique structural behaviour and electronic properties of olivine LFP structure were found to be directly linked to changes in the bonding character, which varied significantly with micro-strain. We propose that the observed lattice expansion in LFP is due to the weaker hybridization of e_g states with oxygen. The effect of micro-strain on the electronic properties of LFP is reflected in the observed enhancement of polaronic conductivity by an order of magnitude that is highly beneficial for improving the performance of electrode materials.