Molecular and Electrode Arcitectures for Multivalent Conversion Electrodes
Authors: Nanda Jagjit , Zhou Hui , Martha Surendra K , Li Juchuan , Dai Sheng , Pannala Sreekanth , Wang Junjie , Braun Paul V , Dudney Nancy J , Adcock Jamie 
Published: February 2020

Multivalent conversion-based binary transition metal (TM) compounds have recently gained significant attention due to their higher specific capacity compared to conventional intercalation-based lithium-ion materials, due to their multiple red-ox states contributing towards transfer of > 1 lithium per transition metal atom. 1-2 However, harvesting reversible multi-electron capacity from multivalent systems presents many challenges, such as poor electronic conductivity, poor transport and interfacial charge transfer kinetics, and structural instability during multi-electron charge transfer, which leads to poor cyclability and huge voltage hysteresis during the charge-discharge cycles.3 Many approaches are tried by us to solve these issues. Initially, we investigated the role of electrode architecture on the capacity retention and hysteresis.4 A electrode architecture consists of nanometer sized iron fluoride particles (~25-50nm) coated with multilayer graphitic platelets and bound to an electronic backbone comprising of an interconnected network of carbon fibers (5-9 µm diameter) was specially designed. This unique combination of reduced particle size and electrode architecture significantly improves the local electronic conductivity and enhances the electrochemical performance such as better capacity retention and C-rate performance. At the same time, the voltage hysteresis was significantly reduced from 1.5 V to ~0.9V, compared to normal slurry electrodes. The reaction transport kinetics was further improved at elevated temperature (60oC), yielding almost theoretical specific capacity (700 mAhg-1) with reasonably good cycle life (Figure 1). We also explored alternate 3D architectures through a colloidal templating strategy to produce inverse opal scaffolds for fabricating iron fluorides as well as oxides electrode that could provide a bicontinuous channels for facilitating ion and electron transport, to greatly improve the kinetics and voltage hysterisis.5The overall performance of such 3D templated structures are impressive, and may provide a new architectural concept for next generation high energy density batteries. In addition, we proposed a novel molecular approach for conversion compounds by converting iron oxides to oxyfluorides/fluorides through a controlled fluorination process.6 Our study suggests that the fluorination process occurs from surface to core of the oxide particles with increased fluorinating temperatures. This is supported by electrochemical capacity enhancement along with a concomitant increase in voltage plateau of the fluorinated samples as demonstrated in the electrochemical cycling (Figure 2), differential capacity and cyclic voltammetry results.

Acknowledgement

This research is sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy. Electrode and materials characterization work is supported by Vehicle Technologies Program for the Office of Energy Efficiency and Renewable Energy, Department of Energy.
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