Elucidating the Origins of High Capacity in Iron-Based Conversion Materials: Benefit of Complementary Advanced Characterization toward Mechanistic Understanding.

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ConspectusLithium-ion batteries are recognized as an important electrochemical energy storage technology due to their superior volumetric and gravimetric energy densities. Graphite is widely used as the negative electrode, and its adoption enabled much of the modern portable electronics technology landscape. However, developing markets, such as electric vehicles and grid-scale storage, have increased demands, including higher energy content and a diverse materials supply chain. Alternatives that provide the opportunity to increase capacity and address supply chain concerns are of interest.Understanding the fundamental mechanisms that govern battery function is crucial to driving further improvements in the field. Advanced characterization techniques, such as those enabled by synchrotron light sources and high-resolution electron microscopes, that can uncover these mechanisms have become a necessity for elucidating structural evolution upon electrochemical conversion at the nano- to mesoscales. Performing these experiments with relevant electrochemistry using in situ and operando experiments imparts the ability to identify critical reaction pathways and capture intermediate (dis)charge products not discernible by traditional experiments.This Account describes a series of recent studies focused on the advanced characterization of spinel-type iron oxide-based anode materials. These studies begin with magnetite (Fe3O4), a low cost iron oxide which, when synthesized with appropriate coprecipitation based crystallite size control, provides opportunity to realize eight electrons per formula unit via electrochemical reduction. We then transition to bi- and trimetallic ferrites (such as ZnFe2O4 and CoMnFeO4) and conclude with high-entropy spinel ferrite oxides (HEOs) that contain at least 5 transition metals. For each material type, a variety of characterization techniques are utilized to describe the fundamental reaction mechanisms and rationalize electrochemical behavior. X-ray absorption spectroscopy (XAS) is featured prominently, as it allows for element specific analysis of electronic structure and local atomic environments, including nanocrystalline products of electrochemical conversion. Combining XAS-based techniques with diffraction and microscopy, the transition of iron oxide-type electrodes from spinel to rock-salt to metal nanoparticles upon full lithiation can be deciphered. For magnetite and its bi- and trimetallic ferrite analogues, delithiation results in return to a highly disordered network of FeO-like domains. Notably, while magnetite and the bi- and trimetallic ferrites appear to be limited to reoxidation of Fe to the 2+ state, through introduction of entropy-induced structural stability, higher Fe oxidation states (up to 2.6+) can be accessed upon electrochemical oxidation. These materials may hold promise as alternatives to traditional graphite electrodes where their combination of high capacity and compositional flexibility provides an avenue toward low-cost, sustainable energy storage.