A New High-Valent Fe-Based Redox Couple in Intercalation Electrodes

Layered transition metal (TM) oxides (LiTMO₂) are attractive positive electrode materials for use in portable energy storage applications such as electric vehicles, primarily due to their high energy densities and their robust charge-discharge behavior. However, the persistent use of TMs such as Co and even Ni, which are both limiting in their cost and abundance, will restrain current technology from keeping up with the rapidly growing demand for portable batteries. There is, therefore, a great incentive to incorporate inexpensive, earth-abundant elements in commercial batteries. Iron is the perfect candidate, given its availability and low mining/refining cost. However, high-voltage Fe-based redox systems based on the Fe^3+/4+ couple always exhibit simultaneous anionic redox and O₂ release, and as a result, always display highly hysteretic charge-discharge behavior.<br/><br/>Surprisingly, the compound Li(Li_1/3Fe_1/3Sb_1/3)O₂ (Li₄FeSbO₆, referred to as LFSO henceforth) does form a layered system and exhibits remarkably stable electrochemical charge/discharge characteristics with &lt; 0.2 V of hysteresis. LFSO forms a layered structure, with Li⁺, Fe^3+, and Sb⁵⁺ sites having honeycomb ordering in the TM layer. Upon electrochemical cycling, this material exhibits a voltage plateau at ~4.2 V, which is stable for multiple cycles and corresponds to an extraction of almost 2 Li⁺ per formula unit during the first discharge. Several details about the electrochemistry of this material are striking. Firstly, the discharge potential is 4 V vs. Li⁺/Li, which is greater than the reported average potential of conventional NMC-based systems (~3.8 V). The first cycle charge capacity is close to 175 mAh/g, which is similar to the theoretical capacity of LiFePO₄ (165 mAh/g, operating at 3.2 V). Many questions remain regarding the mechanism behind the electrochemical performance of LFSO. For instance, the nature of the electronic charge compensation that accompanies (de)lithiation is poorly understood. Additionally, the structural transformations that accompany the redox plateau have not been thoroughly characterized. Investigating the electronic and lattice structures of LFSO is of great importance, as Fe-based redox in layered systems has not been studied before.<br/><br/>In this work, we address this gap using a variety of synchrotron and lab-based characterization techniques. We use X-ray absorption spectroscopy (XAS)-based techniques to investigate the electronic charge compensation mechanism during (dis)charge. We use Fe-L edge Resonant Inelastic X-ray scattering (RIXS) in conjunction with O-K edge RIXS (both of which are known to be sensitive to TM-3<i>d</i> orbitals) to accurately characterize the changes in Fe and O valence orbital occupancies during (dis)charge. Coupled with ⁵⁷Fe Mössbauer spectroscopy and charge-transfer multiplet calculations, we present a complete picture of the electronic structure changes that accompany electrochemical (de)lithiation and identify the redox-active species in the reaction. We also perform X-ray diffraction (XRD) experiments to study the structural transformations during the (dis)charge.<br/><br/>Our results show that the 4.2 V plateau is accompanied by a classical two-phase transition, and the electronic charge compensation stems from a novel high-valent Fe-based redox couple. While the presence of Sb in the system limits its commercial interest, LFSO serves as the ideal model system in which to characterize high-valent Fe redox. Some preliminary work carried out into substitution/doping strategies opens the door to achieving low-hysteresis, high voltage Fe redox in layered oxides using earth-abundant elements, which could serve to reduce our dependence on Co/Ni-based positive electrodes.