Ion Transport in Chromite Spinels for Multivalent Battery Applications
Authors: Johnson Ian D. , Mistry Aashutosh , Yin Liang , Murphy Megan , Lapidus Saul H. , Srinivasan Venkat , Vaughey John T. , Cabana Jordi , Ingram Brian J. 
Published: December 2020

Multivalent-ion batteries, based on the reversible reactivity of metal cations Mn+ (where n ≥ 2), offer great promise to supersede the current state-of-the-art Li-ion battery and provide a step change in energy storage capability. Oxide materials based on the spinel structure, AB2O4 (where A is the diffusing ion, M2+) have previously demonstrated potential to be high capacity, high voltage cathode materials for such batteries, with a 3D network of M2+ diffusion channels to facilitate ion transport.1–3 In particular, the MgCr2O4 spinel has been of interest due the predicted high electrochemical potential (~3.5 V) of Cr3+/Cr4+ vs. Mg/Mg2+.2 However, reversible Mg2+ intercalation kinetics has often proven to be poor in such systems, with large overpotentials observed on charge and discharge.3,4 The origin of these kinetic barriers is still poorly understood, where it is postulated that poor Mg2+ diffusion through the oxide lattice, sluggish ion transfer between the electrode and electrolyte, or resistive surface layers from electrolyte decomposition products may be responsible.
In this presentation, we discuss the application of solid-state impedance spectroscopy to a library of doped spinel compounds, M1−x
Cr2−2x
Ti2x
O4, where M = Mg or Zn, to elucidate the diffusion behavior of Mg2+ and Zn2+ in these systems. We reveal that incorporation of Ti had significant impact on the impedance behavior of the materials. As both M2+ and e− were found to diffuse in these materials, they displayed mixed ionic and electronic conductor (MIEC) behavior.5,6 This presentation includes the development and interpretation of a new physical model to describe the motion of M2+ and e− in the M1−x
Cr2−2x
Ti2x
O4 spinel, which was necessary to accurately describe their diffusive behavior and extract their conductivities. This study therefore represents a breakthrough in the understanding of M2+ motion in these systems, and provides a methodological framework for further studies of other electrode chemistries.
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