Multivalent metal-ion battery systems have the potential to provide cost-effective and higher capacity alternatives to current lithium-ion batteries. Moreover, the electrode compositions can include earth-abundant and chemically diverse structures, also allowing for multivalent metal anodes without dendrite formation. In particular, current magnesium and calcium-ion batteries, based on high-voltage oxides and phosphates cathodes, show high energy density but limited cycling stability. One key challenge to achieving higher performance and stability in these materials systems is to understand the fundamental structural changes to the cathode at the atomic scale, such as cation disorder, planar defects, and (de)intercalation mechanisms resulting from the electrochemical cycling. The large ionic size of multivalent cations makes it imperative to gain insights into the structural changes in cathode materials during cycling to realize the theoretical operating potentials and further material optimization for multivalent metal-ion battery systems. In this work, we use aberration-corrected scanning transmission electron microscopy (STEM) to characterize structural changes and chemical heterogeneities occurring during (de)intercalation of magnesium and calcium ions into their respective cathode material systems. STEM enables atomic-scale structural and chemical characterization with high spatial (&lt; 1 Å) resolution, which can be leveraged to describe electrochemistry at the cathode. We will also discuss the implications of these findings on the ongoing efforts in materials synthesis optimization and device performance for multivalent metal-ion battery systems.

Characterization of cathodes for magnesium and calcium-ion batteries at the atomic-scale

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