(Invited) Nanostructured Metal Fluorides as Earth-Abundant High-Capacity Cathodes
Authors: Wang Feng 
Published: February 2024

Current battery technologies use intercalation chemistries that rely on nickel-manganese-cobalt (NMC) oxide cathodes and graphite (Gr) anodes. These materials can store and release lithium reversibly, but they have limited interstitial sites to hold Li. In contrast, conversion-type materials, which typically use less expensive and earth-abundant elements such as Fe, Cu, O, and S, enable extremely high capacity by reacting directly with the host material to form completely different structures. Among various conversion compounds, FeF2, CuF2, and the solid-solution Cu1-xFexF2 are some of the few capable of multi-electron redox reactions, offering high specific capacity and energy density and relatively high voltages [1-4]. These earth-abundant, high-capacity cathode materials have the potential to double or triple the energy density of current intercalation-based cathodes at less than ½ the cost. However, there are some fundamental challenges that have hindered practical applications of Cu/Fe-based fluorides, including i) Cu/Fe dissolution during charging leading to irreversible capacity loss [5, 6], ii) the highly ionic nature of the Cu(Fe)-F bond that induces poor electronic conductivity, hampering reaction kinetics and reversibility [1], and iii) the high mobility of Cu/Fe ions that can cause aggregation and continuous coarsening of nanoparticles, leading to segregation from other components and ultimately disrupting the conductive network [7].

In this presentation, we will provide an overview of recent progress in addressing some of the challenges associated with Cu/Fe-based fluoride cathodes through synthesis/processing and interfacial engineering [3, 8-10]. With specific examples, we will explain how nano-structuring is essential to conversion redox and maintaining local electronic and ionic transport over cycling [2, 4, 7, 11]. Furthermore, we will discuss the fundamental principles and requirements for designing electrodes and electrolytes that maximize energy density, rate, and cycling stability of Cu/Fe-based fluoride cathodes, providing our perspectives on the pathways of implementing these cathodes into full cells for large-scale applications.

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