Study of the Zn Insertion into Nanostructured d-MnO2 in Non-Aqueous Metal Batteries Using in-Situ X-Ray Absorption Spectroscopy

Abstract

Nonaqueous multivalent battery chemistries provide high theoretical volumetric capacity, limited dendrite formation, and wide electrochemical windows that are a promising alternative to traditional lithium-based energy storage materials. However, transport limitations and volume change have often restricted cathodes to nanophase materials. To better understand these structural changes, we have studied Zn intercalation in δ-MnO2 cathodes using X-ray absorption spectroscopy during charge and discharge. When paired with a zinc metal anode and a AN-Zn(TFSI)2 electrolyte, the MnO2 cathode provides excellent reversibility (~100% Coulombic efficiency) and stability for 50+ cycles with ~100 mAhg-1 capacity with an operating voltage of 1.2 V vs. Zn/Zn2+. In situ x-ray absorption spectroscopy (XAS) provides element-specific characterization of both crystalline and amorphous phases and enables direct correlations between electrochemical performance and structural/redox changes associated with Mn and Zn species while cycling. By looking at the main edge of the Mn K-edge XANES spectra, the δ-MnO2 samples show clear reversible changes in redox state upon Zn insertion and deinsertion which allows Mn absorption edge shifts systematically from left to the right while cycling. These redox changes can be correlated to the local structure at the Mn site using extended X-ray absorption fine structure (EXAFS). We find a substantial decrease in the first two coordination shells upon discharge and nearly complete recovery upon charge, which we attribute to Jahn-Teller distortions during reduction to Mn3+.