Tunable electronic structure of heterosite FePO₄: an in-depth structural study and polaron transport

The development of better electrode materials for lithium-ion batteries has been intensively investigated both due to their fundamental scientific aspects as well as their usefulness in technological applications. The present technological development of rechargeable batteries is hindered by fundamental challenges, such as low energy and power density, short lifespan, and sluggish charge transport kinetics. Among the various anode materials proposed, heterosite FePO4 (h-FP) has been found to intercalate lithium and sodium ion hosts to obtain novel rechargeable batteries. The h-FP has been obtained via the delithiation of triphylite LiFePO4 (LFP), and its structural and electronic properties have been investigated with different crystallite sizes. The synchrotron XRD measurements followed by Rietveld refinement analysis reveal lattice expansion upon the reduction of crystallite size of h-FP. In addition, the decrease in the crystallite size enhances surface energy contributions, thereby creating more oxygen vacancies up to 2% for 21 nm crystallite size. The expansion in the lattice parameters is reflected in the vibrational properties of the h-FP structure, where the red-shift has been observed in the characteristic modes upon the reduction of crystallite size. The local environment of the transition metal ion and its bonding characteristics have been elucidated through soft X-ray absorption spectroscopy (XAS) with the effect of crystallite size. XAS unequivocally reveals the valence state of iron 3d electrons near the Fermi level, which is susceptible to local lattice distortion and uncovers the detailed information on the evolution of electronic states with crystallite size. The observed local lattice distortion has been considered to be as a result of the decrease in the level of covalency between the Fe-3d and O-2p states. Further, we demonstrate the structural advantages of nanosized h-FP on the transport properties, where an enhancement in the polaronic conductivity with decreasing crystallite size has been observed. The polaronic conduction mechanism has been analyzed and discussed on the basis of the Mott model of polaron conduction along with an insightful analysis on the role of the electronic structure. The present study provides spectroscopic results on the anode material that reveal the evolution of electronic states for fingerprinting, understanding, and optimizing it for advanced rechargeable battery operations.